I completely agree, there is no problem with what you are asking. Heck I do it both ways. Once I mark the center of the joiner I dry fit it in both wings. If it's hitting the bottom of the joiner box and my center mark is right where it should be I will epoxy both sides at the same time. This is rare and I can count on one hand how many times I was able to do this.

In most cases I find the center of the joiner, dry fit everything, pick a wing and epoxy the joiner into it. I'll let is setup for a couple of hours, then I will coat the ribs, joiner and the joiner box and rib on the other wing then put them together, then wipe off all the excess epoxy and tape them together.

I also have a habit of fiberglassing all of my center sections with 1 inch wide tape, but that's just me as I tend to fly my planes pretty hard so I like the added strength.