I only ever plan my first 2 records (which can change) then I go with whats been played whats not been played... I always think if you plan your set in your frontroom that day.. you may have a 70's head on (for example) and if the dj before you has just played a 70's set and it didn't go down to well.. you need to play something else.. If its half way through the night and no ones played any R'n'B then I play a bit of R'n'B... If the dj before has played a lot of stompers i'll go midtempo... I never plan a set and don't think I ever will, I now have enough faith in my playbox that I hope I don't have to worry and I can sort it as my spot goes on... (my box may not be to others taste but thats a differnt story)
With me I only put in my box what I would play and that are dancers, I don't carry records around I would never play or I don't have faith in... I know people who do this... why?
I always think theres no need to carry a box of 300 around with you... if at the end of the day you wouldn't play half of them anyway.
Mind you, your playbox is a perssonal thing... so each to there own