My favourite poster media is often unavailable in 42" rolls so I pretty regularly cut 50" rolls down using an ordinary hand saw - so long as it's sharp and fairly fine toothed it works well and makes a lot less mess than a powered saw, no problem with depth of cut either. The cut edge isn't perfect and I always cut the left hand end off so the printer is detecting the factory cut edge during loading. The tricky thing is to cut the roll square - I tape the roll either side of the cut and use a piece of A4 paper taped tightly round to provide a guide.