As long as it's a dry day. I can't fathom anybody trying to push the chair around in mud.
As far as the MI Ren Fest goes, you aren't really missing much except over-inflated ticket prices, over-inflated food prices, over-inflated garment prices, over-inflated item prices, etc...., and TONS of tourists.
If you get a season pass and actually use it then it's worth it, if you don't buy anything, just hang out by the tavern, and take in the shows (they usually beg for money on top of the ticket price though, but a couple bucks usually shuts them up if they come your way.) Everyone should see Dead Bob at least once, although it isn't anywhere near as good as it used to be, and it's pretty much all one standard script now so once is enough.
If you do go though, consider donating to the medical fund. http://rescufoundation.org You'd think with all that money they take in they'd get medical care but NOPE.