I do the same for anything I consider important, but max out at 9 years. If you want to transfer to another registrar you need to be able to add 1 year. That means transferring a domain that has over 9 years remaining can fail because of the 10 year max. If you get in a dispute with your current registrar you'll want to be able to transfer to another one without waiting to get under the 9 year mark.