I think simply learning and FOLLOWING opening principles is sufficient. The important part about learning openings is knowing how to punish someone who's breaking those principles (and not following theory). I've never really played e4 in my life in any rated game, online or off, but when I play my 1100-1400 friends I can play e4 or any first move and crush them out of the opening because they either 1) don't castle 2) move the same piece many times 3) too many pawn moves 4) Give up the center or block their pawns with pieces. 5) Get their queen out as a priority 6) drop material to a simple threat . Also, you need to try develop with a threat to slow his own development down.
Once you got those basics down, you'll instantly know when your opponent makes a small mistake in the opening (and it happens veeery often OTB) and you'll know where to look to punish it. (often by simply following the principles yourself). Often the answer to unorthodox line X in Y opening is just to play good simple solid chess.
When you got the principles down then you can learn an opening and its main ideas, and you can fall back on the principles when your opponent leaves theory ( usually after about 4 moves lol). Once in awhile, you'll find someone who actually knows your opening and follow theory with you, then you'll feel like a pro.
As for the advice on playing e4, well I don't know! Most strong players will give that advice, but I think it depends on what makes you love chess. It's probably true that you learn tactics faster, but you can always do tactical puzzles on training sites for that. I know I don't regret not ever playing e4, because what I love most about chess is the abstract and imaginative strategy and decision making part, not the scientific tactics part. Obviously my tactics suck balls, and probably not playing e4 doesn't help. My point is that you can get to a decent level in so many different ways, because there are so many aspects to improve on in chess. I don't think saying : YOU MUST PLAY e4 until you get to 2000. is correct. I'm not 2000, but give me chess tempo, silman's endgame book, lots of free time and I'll get there in 4 months. but whatever, Nimzo's a remarkable player and a nice guy, listen to him. I just needed some time off the exam studying.
p.s. : but do play the kings gambit once in awhile because it's so darn fun
I echo what Nimzo5 said, play e4. When I see that, I think that I have a game on my hands. I never get scared by offbeat stuff, I like to see white squander their opening advantage by playing something like 1.b3. It might be easier for you to remember strange openings, but its also much easier for me to equalize and get out of the opening alive.
I just switched back to 1. e4 after playing 1.d4 for years to help my tactics.