That's simple enough to take care of though. When you put your hydrometer in your sample, give it a hearty spin. This will dislodge any bubbles that might be clinging to the hydro. Once is stops spinning (usually only a second or two) take your reading. Also, if your sample has been sitting for a couple of minutes with the hydrometer in it you'll want to perform the same procedure.
The yeast cells themselves won't affect the reading.