Depending on your helicopter. Some fixed pitch helicopters work by having the blades just tight enough so that they don't fall under their own weight. Others report tightening the blades as tightly as they will safely go and then aligning them before each flight by hand works better.

I fall into the latter category, but have flown an Esky Honeybee FP that flew great using the loose method. So try both, see what works best for you. If you get really heavy shaking during lift off, try tightening the blades and aligning them so that the leading edge of the blades are at straight 90 degree angles with the flybar.