Someone here recommended this site in my tiling thread
http://www.johnbridge.com/vbulletin/Everyone there is so helpful and it's very interesting to read about all the different issues involved in proper tiling. I learnt a lot about cracked tiles and what causes them - basically the tile foundation is very important. The fact that you are having the same tiles crack means there's a deeper issue (no pun intended).
As you are moving, what you choose to do about is a different consideration. If you use it as a rental, I'd probably have a few people come and take a look at the issues, especially if they are in wet areas as there may be more serious issues.
One of the more eye-opening things I noticed when reading through the threads on those forums is that many builders will take short cuts and choose to deal with the issues if and when the home owner come back to them. The guys on the forum have a thread about the worst tile jobs they've seen/been asked to repair, it's quite frightening really.