The more the rounds, the deeper the anonymity. 2 rounds is already crazy good, 8 is like ultra-mode.
I don't think you need to give people the option for less than 8 rounds. People anon'ing their coins won't care if it takes a bit longer to go the additional rounds if it cuts the percentage down by a ton. After all, this is a privacy-centric coin at heart. The difference between 3^2 and 3^8 is HUGE.