how important is it to learn all the opening theory in the Open Sicilian as white?

Najdorf, Four Knights, Dragon, Scheveningen, Sveshnikov, O'Kelly, Nimzowitsch, Pin variation, Kan, Kalashnikov..... (not to mention all their sub variations and sub-sub variations) 

Can't I just get away with playing a sideline as white? That way I don't have to study all these variations that black has? Or is it important to play the absolute best moves to get an advantage? 

I just play into all the openings instead of learning something like grand prix or alapin. 

they're variations in the open sicilian 1. e4 c5 2. nf3 (d6-e6-g6-etc) 3. d4

Most of us don't master all that theory, you choose whether to play a simple system or if you want to learn all of them, then learn one at a time, then the other. Against Sveshnikov and Kalashnikov, study the transpositions between the two, learn the lines that can occur most in your games, so the quick games will help you practice. Against the Dragon he learns the attacking systems with White and then little by little he learns the other ideas, it is not a common defense because the players are afraid of being defeated, but the attacking ideas are difficult to forget. Against O'Kell I think you can take advantage and play 2.c4 you gain space and the ideas are comfortable.
Against Scheveningen learn the Keres attack.

I think I have mentioned the ones that are more comfortable to study, I would choose one of these to learn first, with time you will learn more. The Najdorf has many variants, it studies a system to play with white.
Learn a main line against each variation, so you won't be surprised as often. First a main line against each variant of each system, then in the future it will be easier for you to extend your knowledge if you wish. I don't know if you'll get tired at the end, but this would be the right way, the problem is that the road is long. It can take couple of years to have a decent repertoire if you study openings very often, that's why most of us don't know much about openings .

I forgot to mention that you should create a folder with a database for each line and a database with the necessary subvariants. Also keep in mind that GMs know a lot of theory from books but I think nowadays they prefer to take variants from chess engines, but it's hard to dig through engine variants to know which lines to save to learn and which ones to throw away. That is why you should first choose to study an opening, since it is difficult to even choose how to study a single opening. Don't forget to organize each folder and database, so it will be easier to review your opening repertoire.