I was just going to make a thread about assembling reloads the night before a launch. Should you wait to put the red cap with the ejection charge on till its ready to be launched though? Good post op, thanks.
I have never had a problem with a pre-loaded motor. In fact as most of my launches are in the cold of winter I prefer to do them indoors where my fingers have feeling and there isn't wind causing issues. I even load the BP in advance especially as for some loads there isn't a red cap and I have to use tape and other measures to secure the BP.
One thing I do suggest is to have some type of tray/dish with sides to contain all the parts so that you aren't looking for a lost ring or other small part in the nooks and crannies and dark carpet on the floor.