I'd modify both lines to
because most blank referers and UAs aren't truly blank. They almost always come through as a single hyphen-- just like the one in your Rule.
Has anyone ever met a blank UA that took the trouble to send a referer? You can achieve pretty much the same thing in half the server time by only excluding blank UAs.
Now then. Some people will disagree with this, but I like to constrain rules as tightly as possible. The chances of a robot swinging by "cold" and demanding all your image files are, for most sites, vanishingly small. So you can express the "pattern" part of the Rule as, for example,
(substituting your actual filename extension) instead of .* --with some further tweaks depending on whether directory indexes have already been added before the request reaches mod_rewrite. If they haven't, you also have to allow for requests ending in slash, or empty requests-- meaning your front page in mod_rewrite-speak.
The idea is that if someone is asking for pictures and stylesheets, they are either an authorized robot or someone who has already been allowed to land on a page, so there is no need to make the server continue evaluating conditions.