I think that a lot of people will tell you different things, but here's my experience:
I had a 26 on my diagnostic (no studying, so not too shabby...).
On my Kaplan practice tests, I had: 27, 28, 30, 32, 29 (I got that freaking 29 two weeks before the exam!! ugh)
Then my AAMC tests were 30, 30, 32, 31 - I kept getting 12s on the verbal, 10/11 on the bio, but then 8s and 9s on ps!!! Argh...
Actual score: 33 S 10P 12V 11B
So, for me, the verbal and bio were basically right on the mark, but luckily I did pretty well on physical sciences. That may have also been partially due to luck, because although I thought the physical sciences was tough, it had a section on Young's Modulus, which would have been fairly difficult except that I reviewed it on a Kaplan exam I took just a week before.
I think that scores do tend to fluctuate, but's it's also pretty rare to be scoring 35s and end up with a 28, as well as if you're scoring 28s to end up with a 35. Dont' stress over a point or two though - just practice, practice, practice!!!
- Quid