I almost never said "no" to trading shifts but then in my first job I had to make my own arrangements for anything and nobody would ever trade so I know how much it sucks to be stuck with a shift you really needed off.
At my first job I always got stuck with the shifts that started at 5AM, a long time before the buses started running. Thus, I had to walk 30-45min (mostly uphill) to work in the morning, work my 6-7.5 hours with only one 15min break (for which I was not on the clock). I used to go on one meal a day (and 2-3 cans of Mountain Dew).
what doesn't kill you... makes you more cynical