Is it worth the 1% to you to get the money earlier? If so, go for it. I don't do a large amount of business, so I didn't find that it would help me.
If you and your builder have an agreement, albeit verbal, you would probably want to make a new "agreement" before you start charging for late payments, especially if you have been accepting the final 10% after 30 days all this time.
Instead of getting into a discussion over late fees and holdbacks, why not just add a little more into your estimate to make up for what you're losing? For instance, if they hold back $1000.00 for 30 days, figure the interest on that and add it in to the estimate. It doesn't have to be itemized as such, you could raise your square foot rate 10 cents or whatever it takes to make up for what you're losing.
Even better, do only 90% of the job and tell him you want 90% of the money now and you'll come back and do the other 10% in 30 days. See how he likes that.

10% is usually the standard late fee. At least that's what I've heard. Most of us wealthy tile installers have never had to pay a late fee in our lives.
