Most people will say no, they much prefer dead material to live. They are very commonly used in bio--active enclosures housing all sorts of creatures and are generally thought to be safe. When I looked into them I chose the "rough Slater" species, Porcellio scaber, over the "pill bug" sort (can't remember the Latin off the top of my head, but the shiny ones that curl tightly) because they don't burrow as deeply and the scientific data didn't include live foods at all whereas the pill bugs were listed as being less fussy and more opportunistic and it was thought may eat a pupating insect, but they were just basic observations made by scientists. All that was available to research really were basic "environmental impact" observations of wild populations because of them being an introduced species in many countries. Anacotal observations by hobbyists suggest that either species are harmless when used as clean up crews. When I was (trying to) research them initially it was to house them with tiny communal scorpions, I never had a problem with them bothering moulting scorpions at all, even though they were bigger than the young scorpions that were moulting. I haven't read any stories of isopods harming any living animal at all, I can't imagine them bothering a spider.Cool, thanks Dave.
Would they be a problem if my spider moults?