I've been considering buying a BlocLoc ruler for squaring up small blocks for exactly the reason you state - on small blocks, the ruler always wobbles.
I didn't use my rotating mat much until I took a class on "disappearing" blocks. We made disappearing pinwheels, and being able to rotate the whole mat when doing the second set of cuts was priceless. Don't consider the mat a waste; you might find a better use for it in the future.