There is no safe butane, and there is no dangerous butane.. Whether its pure, or mixed with other gases like propane, the flashpoint will be lower than atleast -60°C..You're best to work in a cooled area well ventilated (with positive pressure if using any fans since you don't want to pull it through a fan sparking at 60x/second)..
Avoid the temptation to work in a deep-freezer to reduce volatility since you're dealing with heavier than air gases, and they can quickly build up to dangerous levels in the freezer itself..
Lastly, don't underestimate the time needed to clear the area after the fact!!