Read the manual
heh
Automatic transient detection is a minefield for programmers, often this can be worse than a bpm-based calculation, so in reality, warping manually is better than either if you want to make sure things are correct.
Only those wanting to save time, or who don't actually care if their tracks are tight as want automatic warping, performance DJs etc would die if they didn't warp properly.
Some of the best webstores offer .ASD files - which are properly warped, and they offer this free of charge, so if you're into some of the more forward thinking dance music, you're in good hands. The rest of the musical universe, well, you're going to have to sit there and warp properly for a while.
I'll say one more thing, there are several time stretching algorhythms, each has it's own quirks. Do some research on what you need, there are ones that are better than others for full tracks (Complex etc) and in Live 8 there are even more options, and you'll need to know a bit about quality levels vs. your processor load before making these kinds of decisions (and if you want to perform live at some point, how many of these you can run before your processor gets hit too much and you get dropouts).
If you're info hungry enough to read our pathetic attempts at explaining Ableton, seriously, go to the pros, they know what they are doing.