I see KJ's point, but I personally like to buy in a little larger. Usually it's 10-40 units, which is what most civilians buy in for on low-limit games. To me, this is a good balance between buying in so large that your minimum bets become conspicuous ("he bought in for $1K and bets $10 a hand?") and having to constantly rebuy during positive counts. It seems to me that many of the times that I need to rebuy in the middle of the hand are when the count is very high, requiring more money for aggressive double downs and taking insurance—exactly the times when you don't want the floor coming over to check out the transaction. It certainly seems awkward when I buy in for $200 betting nickels off the top and the boss swings by to verify my buy-in of $400 to take insurance.
Another factor to consider is the amount of time needed to change bills. This is especially true in games where you need to maximize your up-time (say, if you're flat betting during the 60 minutes that a "good" dealer is on, you might want to have enough chips handy to avoid doing cash transactions until the 20-minute relief dealer comes on, when you actually want to slow down the game). And in a good counting game where I'll be playing the usual short session, I want to play as quickly as possible, and every time I have to buy more chips (again, usually during good counts), it costs me a couple of rounds, which could be 4-8 hands, depending on how deaf the floor person is.