I'm New Mexico ranch raised and still ranching. Cowboys tucking their pants into their high topped boots in these parts has always been common and I remember as a kid seeing old timers with their pants tucked in at the monthly Dunlap dances and the yearly Ice Cream Social. I have also seen World Champion Steer Roper Sonny Davis dragging calves in a branding pen wearing lace up army boots. I also remember one of those old timers always wore a set of OK spurs set above his 2 1/2 - 3" way underslung heels. He was at least 6' 6" tall and mostly leg and I wouldn't be surprised if his tops weren't close to 22 - 24" tall.
Another neighboring rancher (who was born in the 1880s) told me that he use to wear boots with tops so tall that he had to take them off before he could take down his pants to relieve himself.
It has been my experience that tucking your pants in while in brush country (whether you wear leggins / chaps or not) is a sure fire way of filling up your boots with sticks and cedar berries.
I'd say that the general rule here is, if they are low tops (12"), pants are on the outside. If they are tall-tops, pants are tucked in.
There are no trees here and I have chicken legs and as a consequence, I tuck mine in for a little bit more padding, so that the tops of the boots don't rub holes in me when I ride.
Billy
Edited to add... I ran across these pictures on the Internet some where, some time in the past and because they are early pictures and the 1st guy has full scallops, I put them in my file. The boots are still square toed and I'm not sure when people started wearing round toed boots. I don't really think this fellow paid for full scalloped boots to muck out stalls. Its style.
And for those wondering about exotic leathers, check out this guy's pants.
And edited again:
1961MJS
My Dad (who was a Texan, Cpt. Dan) taught me to take the pants leg, at the bottom of the lower seam, in the middle of the ankle and pull it out until you have a fold running from there to the middle of your knee. Then fold that over to the
outside of your leg, hold it and pull on your boot. That way you have narrowed up the pant leg and have no wad of pants leg between you, the boot and the saddle fender. If the pants are too long, make as much cuff as you need first or they will wind up baggy above the knee and wrinkles might eat on you before you get out of the saddle. There is nothing wrong with using talcum powder liberally. And I've seen guys pull their (long) socks up over the folded pants leg to hold the pantlegs in place. And, I have shortened long pants by just whacking them off with a knife or scissors, because who is to know? Your pants are tucked in every day.
I use to order Paul Bonds and the measurement around the calf would include the folded pants leg.
I wear Wally Mart long cotton tube socks and the newer, the better. (our gyp water tends to harden socks) Again, for padding.
But after reading and writing all of this, I think Johnny McCrae has the best idea if you are shooting.