Never practice against level 9 computers or in Very Hard mode, for reasons you already stated; they have ridiculous reaction time, but on top of that they are extremely predictable and you can abuse the crap out of them for easy wins 99% of the time. Just see a couple videos on youtube of decent people facing level 9s and it's easy to imagnie that it really doesn't prepare you for actual competitive play. Just go up against a level 1 computer, or training mode if you need to practice advanced techniques like wavedashing.
On the other hand, wavedashing is gone from Brawl as far as we can tell, L-cancelling is different, aerial speed is different (characters like Fox don't even fall too fast), and we don't know what other techniques will be available to us as we get our hands on the actual game, so practicing Melee in the hopes of getting a leg-up on the competition for Brawl doesn't seem like a great idea. =/