I'm not sure what the difference is between booking and prebooking. Getting rooms depends on the time of the year or the time of the week etc and may depend very much on local factors, eg is there a convention on in town, is it school vacation etc. Greyhound buses often stop in the slightly dodgier parts of town, where you mightn't necessarily want to sleep. Sometimes you get towns with a million motels, sometimes you can't find a bed anywhere. If it was me I'd do some planning ahead and book some rooms somewhere you can get to without difficulty.