According to your heading you had issues with large save files, but 90 MB is not large, not at all. I think that's small. In my opinion it would have to be more than 500 MB in size to be called large.
I forgot to explain what the TravelDB file is in my previous post, so I'll do it now: It's where all the little images go when a sim gets a memory. Before the option to turn off memories came with Seasons expansion, this file often became horribly big, like mine did. After I bought Seasons I immediately switched off memories for good, and nowadays I only have small TravelDB files, and therefore no need to delete them anymore. If your TDB file is only 3.14 MB, you don't have to bother deleting it, because that won't make any difference in your game.
If you're worried about if it's safe to delete this or that file or this or that folder, do like I always do before I start experimenting: Make a backup of your entire The Sims 3 folder. After that it doesn't matter what happens or what you do, you could always go back to the original folder, and everything will instantly be back to normal again.
This thing that your BF said, about Alt+Tabbing to desktop worries me a bit, because I do just that occasionally, (or just press the Windows key, which has the same effect) when I want to check how much RAM the game is actually using at any given time. So far I haven't noticed any negative effects from doing this, but who knows, it might be true. In the readme file of The Sims 1 there was some mentioning of "not using Alt+Tab too often when playing", or something like that. I don't think I have seen it in any of the Sims 3 readme files, though.