In my experience, guitar amps don't sound good. They tend not to have enough range for an accordion. A keyboard amp is better. I have a big old Peavey that I used to use, but it's so big, I tend not to haul it around any longer. Should probably get rid of it. I do have a Carvin acoustic guitar amp that I use occasionally. Sometimes I use it as a stage monitor because I get better control than I would if I only used the PA monitors.
But, having said all that, the reality is that the vast majority of gigs, I just go into the PA and rely on the PA monitors. They are all there anyway, they sound good enough, and it minimizes the amount of junk to haul around.
For rehearsals, using the PA works for me too. I only use internal mics any more.
I found an awesome solution to my quandary. Since I rehearse with several musicians in different situations, I purchased a Roland AC-60 acoustic amp. Holy catfish, this thing is amazing! What a rich and authentic sound it produces. It can be used as a stage monitor too, so I got a tripod speaker stand to put it on. It has some amazing technology built-in... no feedback. The entire amp is about the size of an Opelousas air-compressor! And the lows rumble while the highs sizzle. It's incrrredible. Did I mention with NO feedback!? It's the best "AMP" I've ever found for a Cajun box (internally mic'd and externally.)
I highly recommend this product to anyone wishing to jam or rehearse with a band in a casual setting or would love a little ear-level monitor at a gig (give your knees a break!)