Hey, I know this must have been asked several times, but I have a lot of questions regarding the LTWs.
What is the safest way (which does not cause neighborhood/game corruption) of changing a Sim's LTW?

While using and changing the Sim's aspiration, the LTW didn't change, by the way (I just tried now).
And, what is the safest way of deleting an object spawned by this cheat (specifically, the Sim Modder)?

Or are there any other ways of doing this?

To change a sim's LTW, it's easiest to either get the lot debugger from MATY, or the simblender from simbology. Both should have options (IIRC) to alter LTWs (can't remember specifically at the moment).

As for deleting objects spawned by testingcheats, I believe using the regular 'delete' key is fine.

I use the sim blender. You can also use the college adjuster by twojeffs. The blender has two options, to randomly choose a new one, and to cycle the possible ones that that sim can have (this is determined by aspiration unless you have any mods affecting this).

You can also change it in SimPE but be careful, it's possible to change it to any normal want and some of these are not designed to be LTWs so could possibly cause corruption. But I believe people have changed the numbers around without any trouble (e.g. Have 10 grandchildren rather than 6, etc)

Other than that, though, I don't think that changing a sim's LTW is ever game breaking is it?

To use the batbox (lotdebugger), click it and choose "wants/cycle LTW" or "reroll LTW." Cycle will go to the next one on the list of available options, allowing you to go through one by one and stop when you have one you like. Reroll will give you another random generation.

There are mods that specifically address how LTWs are assigned, which you might want to look into if you find yourself changing the LTW very often.

Deleting items spawned by the testingcheats is fine. They're just like catalog items.

I just use the bat box and cycle LTW. Sometimes I use the Sim manipulator to change aspiration and roll until I get what I what and then change the aspiration back to what I want.

You can also use the Sim Modder, but that option will cycle through all LTWs instead of just the ones reserved for that aspiration, which can take forever.

Only if you click 'Cycle through ALL lifetime wants'.

I have always used the sim modder to give my sims 3 extra wants, all the want locks and whatever lifetime aspiration I want for them.

I'm not big on downloading mods, so I usually use the Sim modder to change lifetime wants. I don't think it causes any kind of corruption that I'm aware of, and you should be able to simply delete it after using it.
To change the LTW, I think it's under "Wants and fears" > "Lifetime want" where you can cycle through ALL lifetime wants or those specific to the selected sims aspiration.

I just downloaded and installed Squinge's Any LTW for all aspirations mod so that I could do that. So I could have done it with a built-in cheat object all along!

With only one EP (Nightlife) my choice of LTWs for each aspiration was far too limited, so I felt I needed something to give me a bigger choice, and of course I thought "mod" rather than "boolprop TestingCheatsEnabled". I think to a lot of us those objects you can spawn with boolprop TestingCheatsEnabled are a bit of a mystery. For example I used to think that Rodney's Death Creator must be a mod by a modder called Rodney. (I suppose Rodney really is/was a programmer with Maxis.) Are these objects and their uses described anywhere? I'm always wary of clicking on anything I don't understand!

Try googling the name for it, something useful will usually come up, often a forum post.

I like ltw_variety from MATY for giving variety to the lifetime wants without making them all available to all sims.

If you do not want download any mods, there is aspiration reward in game, ReNuYuSenso Orb. If you use it twice, changing aspiration to another and back, it will change LTW.
Also resets all benefit points. Make sure you use it with high aspiration level. Also you can use cheat box to get more aspiration points and set aspiration level. Commands are listed there if you type "help"

When I tried that, I couldn't assign an LTW to my Sim at all! That's why I changed to Squinge's mod. I don't know what ltw_variety was trying to do, but, with it in place, whether I clicked Cycle LTW or Reroll LTW on the batbox, it just removed Romance teen Jack Gill's existing LTW and wouldn't give him a new one. Perhaps it just couldn't find one that was suitable for him. Certainly none of the career ones seem appropriate for a Sim, who's never done a day's work, and never will do one if he can help it! Maybe J.M. Pescado has never met a Sim quite like my Jack, so his mod just gave up on him!

With Squinge's mod in place, I gave Jack 50 Dream Dates as an LTW, which I'm sure he can manage, maybe even while he's still in his teens.

But it's interesting to think I could have done the same with boolprop TestingCheatsEnabled and the Sim Modder. Until very recently I thought the latter must be a mod like InSiminator, the SimBlender or the Sim Manipulator.

What I would like instead of this hit or miss rolling is a mod that opens a panel and you just click and choose any LTW you want, now that would be handy.