Yep plan the times that you want, then log on ON TIME to select them. I have missed out on timeslots even logging on 2 minutes after tute reg opens.
If you miss your time, you can always apply for a tutorial swap (although they'll usually only grant them if you have a subject clash), or keep logging onto PWeb a few days after tute reg, sometimes people will drop a subject and there will be one spot in a tute that was full before.