Image is important, exceeded by performance. Customers will see the work performed everyday, and only see the crew when they are working.
Talk up the crew to your customers a little bit. Let they know that you are happy with the crew as they are dependable and reliable. You are happy to have them, because they make your company a good company.
I like to introduce the crew briefly to the customer if they should need to talk to me while I am with the customer. I think it shows your pride in your crew, and that you don't have a bunch of losers that you need to keep away from the customer at all cost.
Emphasize to the crew that in addition to the good work you see them doing, that image help to keep you all busy working, and earning. Thank them for being professionals, looking and acting like professionals around the jobsite and customers, and they can screw around and make off-color jokes offsite.
As someone said, lead by example, which I suspect you do, by your concern of appearances. Clean shaven every day, or almost everyday, shirt tucked, etc will show them what you are looking for.
We do dirty work, so I don't think that everyone will look as clean cut as doctors or chefs.
You have uniforms, good equipment, signage. So long as you are doing good work, and good customer relations, I imagine you have most of your bases covered.