I heard soil was an alright tank bedding for a mantis which makes sense since the crickets are hiding in the dead grass I have in there. Anyone have a suggestion?
I suggest you use the search feature. Basic questions like yours have been answered many times. =)
As for your question, I have used pure sphagnum moss, peat moss, and potting soil in the past. I tend to like the sphagnum moss the best. You want your substrate to retain water well to maintain the humidity within the enclosure, so dead grass isn’t exactly ideal. Other substrates are acceptable, but I like to keep it simple.
Your choice of substrate is up to you. Personally, I don't think substrate is necessary for most species. I use paper towels because it's cheap and easy to replace.