Some schools value it more than others. Look at table 11 & 12 in the attachment. Also at the bottom of the tables it has some notes for other important criteria specific schools look at.
I know a few pretty amazing dentists that wouldn't be able to put a sentence together without a myriad of grammatical errors. Yes they went to school outside the states and did international programs here in the US. Though I think this is irrelevant to your situation and question. I do believe that you have to be able to effectively read and absorb information in school in order to eventually graduate. Even though everyone does there studying and reading differently, the RC section is a decent way to check that you can stay focused for an extended period of time and interpret new information
If you have a strong GPA and A's on your English classes in college, then a 18 RC is 100% perfectly fine.
If RC is lower than an 18 then it may fall under some cutoffs at some schools.
I had a 21 AA and a RC that was higher. I'm not sure if it was helpful, but nobody really mentioned anything other than my AA at interviews. I got the impression that schools weren't too worried about the subsections, provided they aren't under 18