I've tried, for the T25, configurations of 8, 12, and 16. I like 12 the best; 8 requires more change making, and oddly, 16 seems to as well. It seems to me that when they have 16, players use those chips a lot. I personally tend to make a bet with the fewest chips. When we used 16, most games I wound up with several stacks of 25s and 100s. That's because when a bet was 350, someone would put in 14 T25s; I'd just put in 100x3 + 25x2. You only have to win one or two hands like that to have many of those smaller chips. I've also played in a game where you got 25x4 and 50x4 for a 25 base. That was not a good way to do it. There was way too much change making, and too many color ups.
For the 100s (almost all of ours are base 25, but sometimes I use base 100), I've used 7, 8, 12, and 16. I like 12 the best because when the smallest chip is T100, you get the same issue as above. However, when we were using 7 or 8, we tended to color up with the T100, putting more on the table. If we had 25x12, and they got 3 more T100s in color up, it seemed to be about the right number of chips.
BG is a fan of not coloring up except with much larger chips. At first, I didn't like that system, and my chip sets weren't designed that way, but they are now. I generally agree with him on that. It is more efficient in play because you aren't adding chips on that will soon come back off. You also don't need as many chips in the set to accomplish the same thing, though it might not matter if you are doing 7 or 8 and adding 3 or 4 vs. just starting with 12 when it comes to purchase, but it does make the game go faster to have them on at the beginning.