if you're practising difficult passages (lead lines, or maybe lightening chord changes) then i recommend sitting until they are perfect or pretty fu*king good.
i was learning the Crazy Train solo and i can do it prefectly sitting down, but when i started playing it standing there were a few kinks. but i just worked on it until it was perfect standing and sitting.
if you're playing with a band, you really need to stand. and you REALLY need to stand during live shows.
i was recently learning a tough Metallica solo and i started standing up and it was pretty messed up. then i sat and got it down, then i stood up.
also, go over as many techniques as possible when you practise.