Yes, you need some extra funds for back bets below 2$. I cannot tell if 10$ is enough, this depends on your markets (how many selections, distribution of odds), your stakes and your strategy. If you keep 2$ extra funds per selection, you are save.
If there are only 5 horses, 10$ is fine in any case.
If you increase your stake, the propability for getting below the 2$ limit is less likely, and you will need less extra funds with higher stakes.
There are rumors about betfair closing accounts when betting below the 2$ limit, so it is a good idea to have some of your bets above the limit.
Martin