Obviously, taste and correctness matter.
It still won't be a good idea to say "The reserved bit must be zero on
send and must be ignored on receive," arguing "Well, we don't want to
use MUST because some implementations don't do that so it can't be
normative."
I'd write "The reserved bit is set to zero on send and is ignored on
receive" and save the command terms for things where one might think
that there was a reason to do something else.
The point of lower case keywords shouldn't be to allow people to be
sloppy and to avoid normative text to make a false consensus easier.
This SHOULD be about writing clearer RFCs and not having to contort
language when should and must are perfectly good non-normative things to
say.
Yup.
R's,
John