Putting your RMS motors together in the comfort of your own home is a great idea. It allows you to take plenty of time without the pressures of launch-day wierdness that might distract you from doing it correctly. You also avoid wind, dust, grass, hair, and any other crud that can foul up the works and lead to motor failure.
For all sizes of RMS, putting them together a day or two in advance is no problem. If you need to assemble more than 2 days in advance, just back off the end closures about 1 turn each. The only thing you don't want to do way in advance is to put in the black powder ejection charge. This should be done as part of the final flight prep. The reason for this is that if for some reason you don't get to fly, you can slightly loosen the closures to relieve compression on the o-rings and store the motor away for the next launch. You don't want to loosen the closures if you've already put the black powder in place (the result would be very bad). I have made a habit of always making sure the closures are screwed down tight right before inserting any RMS motor into a rocket.
If you leave the launch with any of your pre-assembled motors, put each one in its own Ziploc baggie, squeeze out the air, and seal it up. Then put it into another baggie and seal it up. This will prevent any moisture from causing any swelling of the cardboard components and to keep the motor grains from oxidizing which makes them harder to ignite.
Happy flying, --Lance.
