Correct, in fact because most pools charge a fee— and practically all except p2pool expose to to some risk of theft by the pool operator or to the pool being hacked— they reduce your income somewhat.

But they make payments more stable.  I strongly recommended p2pool— it eliminates the risk of theft and the centralization of network resources.  It doesn't provide as much variance reduction as other pools, though then again, if you're a very small miner you're not exactly counting on a payment every day to put food on the table.