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.
Thing is, you could pause darksend as it reach's 4 rounds, use coin control to select the coins with 4 rounds and send those.
Unless you made them undependable until they reach 8 rounds, which is bullshit anyway.
Users can asses the risk involved and make a sound judgment on how many rounds are appropriate.
I like the option of 1-8, infact why not 1-100?