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?

I wear my thickest socks when I try on my boots. I size my hiking boots for that because I usually wear those socks while hiking.

Keep in mind that your feet tend to keep getting bigger as you get older, so be sure to have your foot measured again. I wear my wool socks for that too.

That's where I start, but since shoes are still mostly handmade I still use the forefinger method as a quick check for fit, then I walk in them to see how they feel.