That depends on a lot of things... which retracts, how big the wheels are you're retracting, and what else might be going on with the radio system when you've activated the retracts. Personally, I don't think it's necessary, provided your retracts and servos are adjusted properly. At the end points, retract servos should be drawing essentially no power. When I was testing with the retracts on my H9 Bluenose, just for fun I put a wattmeter on the battery, and a pair of RT88's drew about .7amp when operating. I had a pretty healthy 4 cell pack powering everything, and the voltage dropped down to around 4.6V for about 5 seconds. But if you have a older battery, and are flying 3D manouvers while activating the retracts, or you're going to do multiple touch and goes with the retracts cycling each time, it's conceivable that you could pull the voltage down enough to create a problem.

If I was flying a Spectrum 2.4M setup, I'd run a 6V battery just to avoid a potential voltage drop that might "brownout" the receiver. That will also move the retracts faster, and give you more torque on the servos. The other alternative would be to add a second battery running in parallel with the main receiver battery. That adds capacity, and is simpler than figuring out the circuitry necessary to power the retracts with a separate battery.