I use paper towel for most of the vials I keep my younger mantids in. I do use the coconut fiber almost exclusively for my larger mantids, however. It really holds the humidity in the cage which is so important during that final molt. It also make a more attractive substrate than paper towel. Though it is very mold resistant, any small pieces of cricket or other feeder insect that fall onto it will mold quickly. Good ventilation will increase the time things take to mold, but noticeable bits of feeder insect should be pulled out fairly soon.