There's also a 7 second FWD/REV in the beta software, but it can't be accessed by the included remote obviously. You should be able to access the 10 minute skips by using the UI.

From using SageTV I've come to the conclusion that if you're only going to use two times for each direction, 10 seconds and 2:30 (two minutes and thirty seconds) are just about perfect. If something has commercials those two times allow you to skip through them easily and accurately. 10 seconds is perfect to skip back to see that scene or listen to that clip of dialog you just missed. Holding down either allows you to skip through as much or as little of a video as you want, quite quickly and smoothly.

By editing preference files in Boxee you can affect which "keys" are bound to the different sip times, but I've never heard of any way to edit the times themselves. You should mention that to them in email...