My main saddle is a monoflap with basically external thigh blocks (end above my knee). I don’t think about the blocks much, they’re just there. They’re softer and not huge, and also angled a bit, so they don’t push my leg back unnaturally. Last year I rode someone else’s horse a few times in her brand new N2, which has big, much stiffer external blocks that are straighter. I would say those definitely restrict position and would become a crutch for a learning rider because they lock you in. My second dressage saddle is a Kent & Masters with velcro blocks, which I bought to start my 4 year old in. I threw it on my older mare last night to make sure I feel comfortable/secure it it before I crawl onto my 17H baby for the first time. Although the blocks aren’t huge, having them under the flap is a very different feel. It pushes my leg much more off the horse, and makes it harder to drape the leg nicely. I will keep them on for security starting baby, but if I was going to “do dressage” in this saddle I would either remove them or switch them out for a thin pencil roll that doesn’t lift the saddle flap up as much. So, I think a pencil roll, or a smaller external block that doesn’t force your position is probably OK, but anything that forces a position or prevents a draped leg should be avoided, especially before you have a good, independent position.