Roughing them out anytime never hurts. We usually peel heads and do tails about a week and a half or 2 weeks before the shows, but usually not 3 weeks, especially on heifers...depending on the calf, the hair could grow back to about where you started and not give that crisp clean look you are going for.
If we're showing a big bull or a cow/calf pair, we'll shave the bull or cow's head about three weeks out, so that they have that "mature" look. Other than that, we shave heads about a week or two out, and clip on the rest of them everyday starting about a week out.
A good clipper friend taught me that peeling down the neck/shoulder area 3-4 weeks before really reduces a lot of blending work in the area. And, it has been a lifesaver for me, a rookie clipper. Keep in mind that we show Braunvieh cattle and their hair is not like the clubby hair at all. And, when you clip them, there are many different colors that come out, so starting early is a good idea for us just so they don't look all different colors. Most Braunvieh hair is very coarse and thick but doesn't stand up or out when worked with. Others don't have much hair at all to work with so I think we have to take a different approach than the typical clipper with calves with better hair. I hope that helped.
You Have the head, Topline, Tail Head, Butt, Legs, Heart, Underline, Flank, Neck, and shoulders. With the flank you just kind of trim it up and make it all flow good with your underline. With the heart girth you try and make it look as big as possible to make them look like they have better girth