The ideal settings for I-Frame rates- and the other manual config parameters- depends on what you're watching, your hardware and the network performance. There's no single correct answer.
I-Frame controls how often a keyframe of the picture is rendered from scratch. For intermediate frames that display between keyframes, the system only streams data about what is changing in the picture to save bandwidth.
Longer i-frame = less bandwidth, more artifacts and motion tearing when things move on the screen. Might be good for talk shows.
Shorter i-frame = more bandwidth, less junk on the screen for fast-paced stuff. Basketball and Michael Bay movies will benefit from this if you've got the bandwidth and processing power available.
Have a look here, it provides some extra, rather well written information about adjusting that stuff:
There's a newer version of that support doc aimed at folks who use the Windows player and has some screenshots:
Fiddling around with the settings until you get what you want won't hurt anything. If your quality goes wonky, just turn SlingStream automatic stuff back on. Keeping a log as you go of what manual settings you start with and how things look as you change settings might be helpful.
Another thing that might be helpful is to keep an eye on your live streaming statistics. In the Mac Player, open the View menu while holding the Option key, select Show Statistics Window. I forget how to do it in the Windows Player, I think it's Windows Key-Shift-i or something similar. It's a three-key combo.
[Updated] Windows Player: Alt-Shift-i Web Player: Ctrl-Left Click. (I originally picked this useful tip up from the boards at placeshiftingenthusiasts.com)
Hope this helps,