Actually parking out was traditionaly used to keep a horse from stepping out while a "lady" mounted. When riding side saddle with bulky dresses back in the early days mounting could be a rather taskfull event and you did not want a horse that tended to moved about and parking them out helped prevent this. It also shortened the animal a few inches to aid in mounting. Its gone from "nessesary" to show tradition and is a elegant way to show off the horse. Its now primarily just for show. I also see it as a way to show off trainability and it can indeed hide a sickled hocked horse. (being slightly sickled hock is a common trait in TWH). If you don't show then its not nesseary to teach your horse to park out. Unless you just want to. A proper park should have the front legs perpendicular to the ground (straight down placed under the body) with the hind legs placed backwards from the body (not so much it bows the back). An excessivly parked out horse can be strainfull on the animal and will loose its efffect (physicaly and visualy) and will not add astheticaly to the animal.
You do not want a severly sickled hock horse because they will suffer the same things that a non gaited sickled hocked horse can suffer from. Being slightly sickled hocked in a TWH that performs the true running walk can be a plus in the way the animal reaches beneath itself. The hocks should still be of good size, well made and strong. A TWH that is severely sickled hock suffers from weakness and can suffer from stress related problems in the hock joint ( and lower leg). If a TWH with weak, sickled hocks, excessivly long gaskins (and or sored feet) bows out or twists during travel this is not a feature you want for it shows weakness (the animal is compensating for such) and can create gaiting problem soreness, spavins, fractures, calcium deposits, ligament damage and a plethera of other problems.
Some TWH have long gaskins (a little longer than average is good for a nice long under reach) and may stand camped out (not realy parked) to accommodate this feature. A TWH that stands a tad camped out (again not severely) is an acceptable feature or characteristic. If the gaskin exheeds exsessive length then it becomes weak and then the hock suffers and becomes "hocky" in the action. Also the longer the gaskin the harder it is for the horse to lower his croup and drive from the hind end. You do not want this. If the Gaskin is more than 1 1/4 longer than the femur then the horse will be compromised and will have trouble "getting down" in the hind end for that drive. Some ppl will say the horse is to leggy in the rear. Some Gaited horses appear sickled hock but aren not truely sickled because of the common long gaskin to femur ratio. The angle of the hock itself may not be closed at all or very little when measured. Many ppl mistake this for being sickled hocked in horse with rather long (over 1 1/4 longer than femur) gaskins. In non gaited ( or I should say those that don't perform the true running walk) a more equal ratio length of the femur and gaskin will provide a bit more power. A Gaited (again ture running walkers) horse with a short steep pelvis have trouble getting power from the rear. A long well angled pelvis can work harder and has the ability to "get down" than those with short steep pelvis'. A rounded horse will utelize more muscles, get more power from the rear, tire out less, gait better than an inverted horse. A horse (any horse not just gaited types) should push from the rear and not pull from the front. To often I see gaited horses that are tightly contained, pulling from the front with unsquared, hurried gaits and just plain miserable. A thing I to often see with "shot gun" trained animals.
All horses regardless of dicipline, breed, temperments (high strug or laid back) should stand quietly while being mounted. I have ridden highstrung Saddlebreds (mostly made to be highstrung by artificial means) that were taught to stand while being mounted. Its all about training.
"The question is not, can they reason? nor, can they talk? but, can they suffer?" Jeremy Bentham