If you have 5 gallons in a 6.5 gallon fermenter, are brewing a "normal" gravity beer (say 1.060 or less), and keep the beer temperature at mid 60's during active fermentation, you probably won't have blow-off. There is some disagreement about that - ymmv. But if you control temps so that at least a blow-off is rare, you can start off with the lid loose, with a 1 - 2 pound weight on it. That keeps microbes and fruit flies out, but prevents a violent blow-out that could involve kraeusen on the ceiling and walls. After about 5 days, seal the lid.