I ALWAYS use an mounting block because i believe its best for my horse's back. If no mounting block, I'll put her down in a ditch or I'll stand on a tree stump etc. Its not because I can't get on her without a mounting block - i can - i just think its better for the horse not to have all that twisting action going on with his/her back. Plus, it can't be good for your saddle either if you're not especially good or fast at mounting.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare