I always use glass at least on top, preferably both sides. This is esp important when using a neg that has areas requireing lots of burning (eg overdeveloped negs or those with dense areas requiring burning in. I guess the denser the neg, the longer the exposure, the hotter it gets, the more buckling it does. Sometimes glass is essnential to getting sharp images across the whole image. It aint that hard to keep clean unless you are doing serious volume and changing negs every 5 mins. If you print each image as a stand alone wotk of art, the 2 extra minutes is irrelevant. Glass has saved more time than it has cost for me, but the again I print low volume.