I've found most of my clients don't really care what I wear, but are more concerned with how quickly I can fix their issues.
I usually find myself being called in after someone else fails, and the client starts asking around for a replacement for their current IT... So, at that point, they're just concerned about results, not appearance.
However, on that note, when I meet with new clients, I try to dress a little nicer, and I might even shave.

It also depends on the client and the job that is to be done.
As mentioned by others, cabling or crawling around under desks will almost always get me into jeans. Consulting or "Formal" meetings with clients, or their vendors, and I'll break out the Khakis.
Always logo'd shirts, usually polo, sometimes T-Shirts.
It can get well over 100 here during the summer, which actually gets me into jean shorts...Does that count?
I've found that customer referrals, our reputation and word of mouth out weighs any dress code. I mean, take a look at Best Buy. They all wear khakis and logo'd polo shirts. The Geek Squad trolls wear button up shirts and black khakis or slacks... Does that improve anyone on this site's opinion of them?
I didn't think so.
There's a time and place for dressing professional, and knowing when is pretty much common sense.