No skins are "needed" -- they simply are there.
The skins are here only for player enjoyment, and keep in mind the game also has an option to not display any of them, should players choose.
You can't just look at a plane's skin list and say "oh, that needs more skins!" because it's not always the case.
Granted, there are some I'd like to see, but just skin what you like to skin for. I started in 109s because I liked the plane itself. I also did some 190s because I liked the markings. I'm doing some other stuff lately, but I still want to go back and do more 109s/190s for those same original reasons.
So, skin what you like. If it strikes your fancy to do one plane, do it! You don't need to spread out until you *want* to do a new (different) skin, and then it's just a matter of learning as you go.
It's not like you are building a resume or anything

"Yeah, your USAAF stuff looks good, buy you're lacking in navy planes, and your bombers are just nonexistent!"