I've got no technical advice for you, but just about every phone company or whatever, if they charge for things on a time basis, have some notation of what the "to-the-nearest" unit is.

For example, one phone company here in Australia advertised that its phone plan calculated calls to the nearest second, not to the nearest minute, as with a competitor.

So if you're going to do what you outline above, I think you owe it to your customers to spell out that the service is "calculated in increments of ten minutes" or "to the nearest ten minutes" or whatever.

For instance if I log on six times today, for one minute each, you're going to charge me for an hour. I might be unhappy about that...