Rounding down is not a good solution to the rounding problem. As a software engineer, I also used rounding in my contracts when I was working as a contractor -

Rounding down is not a good solution to the rounding problem. As a software engineer, I also used rounding in my contracts when I was working as a contractor - and here's why: If I have 5 clients at a time, and each one has a concept/idea every day that takes 20 seconds to explain to me (and isn't on the current iteration) then they are distracting me by not managing their time properly and calling me a dozen times a day to tell me about their thoughts. If it is a change to the current iteration, then it should have been discussed when we agreed on the current iteration's feature set. Either way, the call is not a result of well thought out time management. As much as I like hearing new concepts and ideas, I also have to take attention away from a project that I'm working on in order to provide my full attention to the client calling me. After the call is done, I also have to come back to the project at hand and hopefully I'm not working on something that requires that I retain a super complicated thought chain which may or may not have been lost in discussion with another client - especially in consideration that I'm not going to bill on other client project for the time that I've spent having been sidetracked and/or getting back to where I was before the call was made. So, charging in $15 increments causes the client to actually manage their time with the same effectiveness that they would hope that I am managing mine. Rounding down makes this more of a problem for me, not less of one. Now a 90 second call is at no charge, and I can get more than one of those in one hour - still at no charge - based on the suggested agreement.