National Ensign should be flown from a staff on the stern. On a gaff-rigged boat it is more proper to fly the ensign from the peak of the gaff and it is OK to fly the ensign 2/3 of the way up the leech of hte mainsail on a marconi (triangle) main....however, ONLY if you do not have a backstay, if you have a backstay then the ensign will get fouled if flown from the leech of the main. Although "tolerated", it is technically not correct to hoist the ensign on a halyard attached to the backstay. Because, again..it could foul with the sail. However, like far too many traditions of the sea.....Flag flying rules have become more casual over the years....... and so flying an ensign off the backstay may be acceptable now. So many sailors fly their ensigns this way, only a die-hard traditionalist like me would probably object......... so, I wouldn't sweat it....if it works for you....... I wouldn't change it. Deep down, I object more to a boat WITHOUT an Ensign, or with a too small one, than if it is flying off the backstay. At least I've never (that I remember?) seen a sailor flying a too-small ensign off the bow staff of a sailboat! I long ago lost count of how many powerboats I've seen doing that!
Just don't EVER, not EVER, fly the US National Ensign from the spreader! That location is reserved for the "courtesy" ensign of the foreign country you may be visiting, although Yacht Club Burgees belong at the masthead, the starboard spreader may be OK, and If you are a member of the USPS or USCGAUX, it is OK (not ideal. but OK) to fly those group's ensigns from the starboard spreader (not 100% correct, but I can't say anything........ my USCGAUX Ensign flies from my boat's starboard spreader!)
Whether you choose the National Ensign (standard 50 star US Flag) or the US Yacht Ensign (anchor and 13 stars instead of 50 stars on the union), one thing is be sure to get the correct size, at least 1" on the "fly" (long dimension) for each 1' of overall boat length, Up to 18' boat, get the 12" x 18", for 18-24' get the 16" x 24" ensign, and so on. I lke big ensigns, so I have the 16" x 24" on my little DS II.
Oh, and to answer the question about 24/7 flying, or take them down when you go ashore. I was taught to always take down the ensign(s) at dusk or when I go ashore. If I'm not on the boat (unless I'm coming back that same day) the ensignes are taken down and stowed below. Some owners do leave a burgee or aother small flag flying at the masthead 24/7 to keep birds from roosting there, a "private signal" or YC or Club Burgee could be used there. By stowing my flags while not aboard the boat, I have been able to get many year's use out of them before repalcement is needed, saves $$$$!