Thank you so much everyone for all of your work to this thread! I honestly thought it was going to be something simple with a couple responses here and there but it turned out to be a great discussion. It just goes to show how much there is to learn for everyone in the horse world.
We have 4 mares, each one is different. Our "milk mare" Fras who still lactates and her last baby is almost 8 years old, will let you clean the nasty tar she gets all day long. She sticks her neck out and wiggles her lips in ecstasy. The only time Vida has attempted to kick me was the first time I tried to clean her girly parts. I finally figured out if I go in from between the back legs, she's fine with it. Go in from the side and your looking for trouble. Same with her daughter Saro. She lets me go in from between the back legs but if I go in from the side she has a squealing fit. Shiloh, the milk mares daughter won't have it either way. I find the young ones don't need it very often but the "moms" get more gunky.
I notice if they are rubbing their tails and they don't have worms, it's usually dirty girly parts.
ETA- by girly parts I mostly mean teats.
I check my mares girly parts often daily since Gypsy usually asks me to itch her down there. I'll use baby wipes if Gypsy don't have any build up, she will get a crusty build up time to time. Most of the time it will come off when I'm gently itching her, other times I will coat the area with a moisturizer cream, let it soak, that will soften it up making the gunk easier to come off. In the warmer months I will use soap and mild temp water and wash her. I like keeping her clean, she's got pink skin and is pretty sensitive. After her areas clean via soap/water or final rub down with a baby wipe, I put on a moisturizer cream. It helps her and I don't want her getting rashes or unconformable, plus she get so itchy too. I never had any issues with cleaning any of her girly parts, she rather enjoys it as it seems. She often stands legs spread during teat cleaning or itching times and holds her tail up on the other, helps free up a hand. She's a goof though and I'm also really gentile when I'm with those areas. I also use baby wipes to clean her face, nose, etc.
I wash my mares tail once every couple of weeks, because I keep it bagged all the time. Whenever I wash her tail I wash her udder. Every time I groom her I reach down and check, if there is aditional cleaning need I wipe her clean with a damp cloth.