Is it normally acceptable to wear rubber riding boots like these:

When competing in lower level dressage?
I won't be showing anytime very soon but I like to plan in advance, because I'm hoping that I'll get to =D
I only have the option of these or some lower-end field boots, at the moment.
Which would be better? I'd prefer if these would work, as they are much cheaper =P
I've read a lot that dress boots are far more appropriate for dressage, but haven't really found anything about rubber boots like these.