We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
They say "Curls for Girls" and "Sun's Out, Guns Out" because, for guys, strong arms are a sign of vigor, health, and fitness to females, and of dominance to males. For gals, toned beats flabby all the time as a sign of health. It's primitive, but physicality matters because we are animals.
Biceps (and brachialis) are a bit more about vanity than functionality, but we want all of our skeletal muscles to be firm and fit. Triceps are much more functional, but what the heck. Good to be strong all over.
I tend to do one or two sets/week of curls more as "maintenance" than as strength-building. Can't do everything in the allotted time, and curls are "accessory." Furthermore, all upper body exertions stress arms so there is no real need to mess with them.
My point here is to mix overhand curls (whether dumbells, straight bar, or crooked bar) with the usual underhand grip. The overhand grip stresses the heck out of your forearms, so it's a good mix.
Grip (ie, forearm strength) tends to be the weak link in many exertions (ie, deadlifts). Another good forearm stressor is max-heavy Farmer's Walks (squeeze those kettlebell handles like your life depended on it). We like functional multi-muscle group exercises, and that darn Farmer'sWalk stresses everything from legs to forearms and everything in between.
Tip for curls - lower you arms to a count of 3 or 4. Don't just let gravity do it. The "negative" is as important as the pull. That applies to most weight work.