All matches related to top 5 (winners semis & losers quarters) and up should ALWAYS be bo5
It doesn't even take THAT much more time considering that Melee tourneys almost always finish ahead of schedule (or at least before the venue closes)
Assume that an average match takes 4 minutes and the sets in winners semis, losers quarters and losers semis all go to the last game
Bo3:
WSF: 6x4 min (3+3x4)
LQF: 6x4 min
LSF: 3x4 min
Total 60 min
Bo5:
WSF: 10x4 min
LQF: 10x4 min
LSF: 5x4 min
Total 100 min
Ofc time spent counterpicking is also a matter, but when you consider that longer sets late in the bracket mean more hype matches and also give more room for adapting it shouldn't really be much of a problem unless you're running on a REALLY tight schedule
edit

It doesn't even take THAT much more time considering that Melee tourneys almost always finish ahead of schedule (or at least before the venue closes)
Assume that an average match takes 4 minutes and the sets in winners semis, losers quarters and losers semis all go to the last game
Bo3:
WSF: 6x4 min (3+3x4)
LQF: 6x4 min
LSF: 3x4 min
Total 60 min
Bo5:
WSF: 10x4 min
LQF: 10x4 min
LSF: 5x4 min
Total 100 min
Ofc time spent counterpicking is also a matter, but when you consider that longer sets late in the bracket mean more hype matches and also give more room for adapting it shouldn't really be much of a problem unless you're running on a REALLY tight schedule
edit
speak for yourselvesWe don't really do this