“Per conversations with most concerned, the problem of too modern hairstyles on male actors in Star Trek regulars as well as both SAG and SEG, has been resolved. Rather than requesting altering of the basic contour favoured by the actor, a simple and easily adjustable change is being made into the sideburns, ie. pointing the bottom of them rather than wearing them square across.

For those doing single jobs on our show, it is easily adjustable via hair growth in a few days, or touching with a makeup pencil, actual shaving unnecessary. Where possible, however, even on such extras, we would prefer the proper job.

This is mandatory for all actors appearing in our show.”
–Gene Roddenberry on Star Trek hairstyles.

“Regarding Deforest Kelley’s haircut for the ‘Star Trek’ series, I would prefer that it be the same style that he had in the ‘Police Story’ pilot, with the additional specification that the sideburns come to a point. Can you come up with a couple of photos from ‘Police Story’ pilot which show Dee’s hair style clearly so his barber will be able to recreate it? Thank you.”
–Gene Roddenberry on Dr. McCoy’s hairstyle.

Source: Star Trek Aliens & Artifacts [Michael Westmore et al.] 2000

Mod note: I didn’t even notice this until I read the quote. The pointed sideburn is present in almost every single male character from TOS right through to VOY (with a few obvious exception like those with long hair or who are bald and so on). Pointed sideburns are a staple of Star Trek along with a distinct lack of pockets on all clothing (another rigid Star Trek rule). However, neither of these rules applies to Enterprise, most likely due to it being set a century before TOS.