I have read that if a boot is properly fitted, you should be able to slid an index finger between the back of the boot and your heel.

Does the above hold true if you're wearing wool socks, or does it apply to the naked foot? If I wear wool socks, I can't (easily) fit an index finger behind the heel with the boots I'm wearing now. But, I can if it's just my naked foot. I'm looking to replace my current boots since they're not wide enough in the toes; should I also bump up the size, given that I always wear wool socks?