I've done this lots of times with shoes. You just have to make sure the cut can't easily spread to allow your toe to pop out. I assume the pressure point was on the outside of the little toe, right? In that case, I would have probably made a vertical slit in the fabric between the little toe and the fourth toe but not all the way to the end of the little toe. This would probably release the pressure but not risk the fabric tearing open on the side of the shoe.
A better approach I use now is to wear thinner socks and to trim the edge of the insert about an 1/8 inch on the little toe side of the shoe. Most inserts wrap up the side of the shoe a little, which effectively narrows the shoe. Thinner socks can also make a big difference in fit in the forefoot. Check out the Danskins (or similar) socks at Walmart in the women's section. These socks aren't much thicker than a sheet of paper and only cost $6 for 6 pairs.