I work for the tourist board in Scotland and would DEFINATLEY advise booking ahead, the amount of travellers who turn up looking for accommodation, who end up dissappointed as they have to take whats left and sometimes where they end up is no where near where they originally wanted. Some b&b's are particularly small and may only have 2 or 3 rooms so if there are 6 of you get booking on Visit Scotland's accommodation website to avoid dissappointment.
I would advise booking at least 1 night in each destination if you want some flexibility, that way your b&b will be happy to offer you additional nights if they are not full or advise you of other b&b's in the area that have vacancies.