Most of the robots I've had a hand in were held together with bolts--then again, most of those robots have used the kit frame.
What 1618 did in 2008 (and 2815/1398 in 2010) was use rivets. 1618 used the IFI kit frame that year and simply substituted a rivet in each place where a bolt was used originally. The
result (click for the big image) was rock-solid, with no problems the entire Chesapeake Regional. (Bumpers likely aided that.)
2815 and 1398 used a custom sheetmetal frame, which naturally I can't find a good photo of this morning. (The next best thing is
this photo.) We had about five 3/16" rivets on each edge of the frames, along with support from running dead axles. There was then a back plate and a belly pan, also riveted together. While I wasn't in the pits as much this year, I can't think of a single report of frame trouble across two teams and five events. And just as a bonus, we were able to rip apart both drivetrains to shake out drivetrain bugs by drilling out ten rivets and undoing three bolts holding the axles in place. (Okay, fixing the drivetrain issues didn't really seem like a bonus at the time...)
I wouldn't consider rivets an automatic decision on the frame, but they're certainly a strong choice each year.