Unless you're dressing like a slob, I think it matters little. I commonly dress in jeans and "polo" or something similar, and have no real hotel status. However, I'm frequently upgraded (without asking!).

I think most hotels know there are numerous large companies where jeans and casual wear are the norm... especially out west.

My belief is it has to do more with having a friendly positive attitude. I see so many people who are gruff at check-in, try to act important, or "talk down" to the clerk.

At nice hotels I always check-in with a smile, say "yes", "please", "thank you", "much appreciated" rather than the slang language I often hear. I usually even start off on arrival saying, "Hello, how are you doing today?" Afterall, they're working, which can be dull at times, and I'm about to be pampered at their hotel. I enjoy trying to brighten their day. In return, an upgrade often appears.... usually to my surprise as I open the door to find a suite!