I doubt that anything is going to fall but when we first went to school on these back in 92-93 they told us that we must put the plug in there when working on them or you will risk the spider gears falling out. I have always stuffed the spacer in there when working on them because I have the spacer but I have never heard of anyone having a problem with it and there were definitely times where I had a replacement axle right there and ready to go so I did not use the plug on those occassions. All that being said, if you are going to leave it with the axle out of it for an extended period of time I would definitely consider it but if you will have the new axle there and ready to stuff right back in then do not worry about it.

Mark