It's a harmless fetish, if there's no harm (your girl(s) really doesn't/don't mind) no foul - no problem.
If you look up foot fetish, you will read that there is some part of the brain that governs sexual attraction that is in some way closely linked to the part of the brain that has to do with identifying feet, or something like that (sorry for the bad explanation, but it's there if you google foot fetish reasons, look for educational sites not porno sites). Some people are really into feet, some sort of like them, many are indifferent. It's apparently one of the most common fetishes. If your girlfriend likes it and you like it, that's all that counts.
I never really understood the attraction to feet but if you both like it then go for it!
I guess whatever floats your boat. I personally would not like it. My dog likes to lick feet and we don't let her. Who wants wet feet from saliva. yuck.