In order to type correctly, I think that this is absolutely a requirement. You will notice that if you go through any free online typing lessons that it will teach you to utilize both of the shift keys. I highly recommend going through touch typing lessons and going through them more than once. I type a lot in my career and going from roughly 45 wpm to 70-120 (largely dependent on what I am typing) has made a huge impact in my life. Good luck.

I've never really thought about which shift I used, but now I notice I exclusively use the right shift key. Not sure why, but even trying the left one seems odd to me. Technically incorrect, I know, but I'd say it's a matter of just finding what works best for you. I'm averaging about 155wpm, using right shift only.

In theory you are suppose to use the shift key opposite to the hand typing the capital key. I for one don't care and bind my shift to a thumb key.

I usually only use the left shift when typing capital letters but I use the right shift for typing the question mark or exclamation. Not really sure if there's supposed to be a proper placement/usage for it though.

It depends on what your technique is. Traditional 10 finger typing requires that the same key is pressed by the same finger every time, if you only use your left shift you wouldn't be able to capitalize A, Q and Z nor use "!".
Some people reserve they left pinky just for shift and type a, q, z or 1 with their ring finger (usually gamers who are used to placing their fingers in WASD). There is no need to use the right shift in this situation.
Also some people prefer to turn CapsLock on or off to capitalize. (Sean Wrona, but he's on another dimension completely)
If you're going for traditional 10 finger typing, then it is a must. If not, you know you and are the only one who can see if it will be beneficial given your actual technique.

I seem to be stuck on 30 -39 wpm and not moving from there, any tips?

You know, for a majority of my life, I hadn't realized there even was a right shift key until my mother pointed it out while we were having a talk about typing. She said she utilized both shifts depending on what she was typing (with an average speed of 110 or so) but I was baffled because I only ever use the left shift key.
I don't think it would be mandatory, though, because I'm doing pretty fine with just one shift key, and in fact, I've tried to get used to both but it just hasn't worked out... haha.

I didn't notice any improvement in my speed , which is roughly 90 wpm. I don't think using the left/right shifts is something you truly need in order to be a fast typist (not saying I am, probably just average compared to the rest of you people, but not everyone wants to type at 150 wpm, XD)

To make using Shift easier, I have a keyboard that uses the customizable QMK firmware ( https://thomasbaart… ).

I have my shift keys set as "one-shot modifier". That means I don't have to hold them while I press the next key.

Instead, I can tap shift and tap the next the key, and it will be shifted, then Shift will turn off.

This way my hand start heading towards the next position it needs to be in even before the shifted-key is pressed.

There is a simpler way to do this, Sticky Keys. Available right out of the box on all major OS (Windows, Mac, Linux).

Dude at least try to learn the double shift technique, if you type that fast imagine how much faster you would type after learning it.

i don't really feel like it'd be worth your time, because you'd only improve by about 10wpm at most. so unless you're really hardcore about reaching a consistent 200wpm or something i wouldn't bother xD

Idk man, i can't see it making him that much faster. it'd only speed up his capitals, and only by a little

Never even thought about this till now. I think that I use the opposite hand to press Shift except if there is a run of capital letters in which case I just hold down the Shift key until the capitals sequence is done.

I only Use one shift key. I exclusively used the left one only, and you can look at my stats. I've tried to incorporate the right shift key but it's incredibly hard to break old habits.

I use one shift key (left shift) as well. I also don't even use the home row (or any normal typing methods for that matter), but according to this site my average WPM is 65 which is well above average typing speed.