IOLTA programs vary from state to state and your inquiry to the Bar Foundation is where you will get the best answer. (Most have a publication they would love to send you).
In general, when you open an IOLTA you implicitly agree that any fee you impose will come out of the interest paid on the account. That's becasue none, not a single dollar, of the funds belongs to the law firm - it all belongs to their clients and they must account for it. The Bar Foundation, however, does not want you to take any fees from the interest because they intend to use the money for charitable purposes! They cannot prohibit you from charging fees however and some will even give your bank a dandy little certificate if you do not!
Personally, I think it's a rip off that is simply too small for most clients to notice; the bar foundation uses interest earned off other people's money for charitable purposes. I do know of a bank in Tampa that literally specializes in IOLTA's as a loss leader to get the other accounts of the big law firms.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.