i like to have the neck off when i am removing or replacing the mouthpiece...heard horror stories of broken necks from the tension put on the horn. as far as leaving them assembled, your only worry would be from the elements. Even though u are indoors, there is still humidity, dust, etc that gets in there and ultimately affects the joints & pads & all that. I had my Bari assembled overnight once...sitting on the stand in a spare room. The next day i noticed a single spiderweb down the bell....i never did that again.