I don't under clip at home. I'm not sure about the smaller minis, but I've found with the 2 mature Shetland stallions (40 & 45" in height) that we've owned and the 2 jr stallions (3 this year - both look to mature 39/40"), they have part of their crest/mane thicker in some spots and thinner in others.
I did some h/j and we pull the mane. With the big guy, who was shown/presented at open shows as a h/j pony in 2005, I started with pulling completely to make him a h/j mane (the length of a dollar bill). To keep from thinning the thin areas too much, I used an old clipper blade as a "rough knife" and shortened the mane with a brisk up & down rubbing motion with the clipper blades (near his withers it was VERY thin, but about 1' long). It didn't friz any more than the rest of his mane that was pulled (his didn't have a tendency to frizz at all - some do). In the thickest area, I parted his mane and put the "top" over to the other side of his neck (had to braid it and tie it to his halter to keep it over for a bit) and then pulled all of his mane out on the bottom part (just like under clipping). However, it doesn't grow back the same as clipped mane does - grows back sparser and "softer" and automatically "trains" down under the rest instead of extending up thru the top portion like a trojan horse. Pulling takes time, as you literally pull only a few hairs at a time. I never got the mane done in one session on a pony, especially if they were only pulled once or twice a year.
In 2 years, his pulled mane grew back out in the "normal" look for a pony/horse with the middle being longer and pointed in a semi-v at the shoulder blades. I pretty much left it alone then, but continued to 1 or 2x yearly pull him in (he wasn't really working at that time) and would pull the thicker underside - which was much easier than had been previously. I then kept it about the same shape but just pulled to thin and shorten it a bit so it wasn't so heavy as he got older.
His normal, natural, pasture locks. He's 5 yrs old.
He's had it somewhat pulled and shortened. It was thickest from mid-black to first quarter of the white. Thinnest at bottom of white and the black at his withers.
Before his h/j campaign/trimming. He had come to us as a 3 yr old with his mane on the left and no amount of "training" or braiding or wrapping or sweating would make it lay to the right, so we just left it alone and showed him with it on the left. Most of the rest of our ponies had their manes trained to the right from "babyhood".
Pulled a bunch but not finished length.
3 months later after a show. 13 years old.
Back to normal pasture locks after a cleanup, basic pull & clip in Jul 08. 16 yrs old.
AJ lived out on pasture and paddocks his whole time with us. When he was body clipped in both 2000 and in 2005, he was blanketed in the pasture when ever the weather went cooler than 45*, but his coat was allowed to grow back in (he was clipped only once per year). The last couple of years he did wear a w/p blanket in the worst months, but he was never stalled. We purchased him in June 1995 and he lived thru July 2012.