If you've spent any amount of time watching or reading music production tutorials, you probably have run into people who offer up golden rules of audio that are never to be broken. I don't think you should 'always' do anything in audio - everything should be case specific. There ought not be any rules, then, either. Occasionally I will debunk some myths or smash some rules with you guys! Up first....
Rule: All of your bass has to be mono.
Everybody has heard this one before. Anything below 250 hZ or so absolutely has to be mono, right? No way!
There is some solid science regarding this myth, but none of it means that we CAN'T ever widen our bass. It is true that humans have a very difficult time pinpointing direction at lower frequencies. When we hear a sound louder in one ear than the other, we can get an idea of which side it came from (as well as phase differences, time delay between left and right, etc). Discerning which direction a sound source came from is known to researchers as 'sound localization'. Humans have very poor sound localization abilities at lower frequencies. This is literally because the human head is only so wide, and your ears are only so far apart. Once you get below 800hZ or so, the wavelength of the sound is less than half of the width of your head. What each of your ears hears begins to be more and more similar and overlapping. Once you get down to sub bass territory, below 100hZ, its virtually impossible to tell which way a sound is coming from based on level differences at each ear. Things sound more mono than they actually are the lower they go. So, it is true that it doesn't really make sense to hard pan your bass or do anything too drastic down low.
So yes, it is a good idea to keep most of the bass in your track in the center with regards to panning, but that does not mean the bass should be entirely mono. I feel like a lot of producers learn to not pan their bass, but along with that assume you cannot create differences between the left + right channels (i.e. they assume they must stay in mono). Not true! In fact, creating some width or side channel information with your bassline can really be of benefit in certain cases.
Let's suppose you are wrapping up your latest track. You have EQ'd away some mud, used some sidechain compression, and chosen your sounds wisely - but the bass is still lost in the mix. You turn up the bass but find it eats up too much energy and ruins the balance of your mix. The solution? STEREO EFFECTS! Without having to boost the gain at all, you can make a blurred bassline pop out of your track by adding a chorus, reverb, or delay to it. These effects make the left and right channels sound different (not mono anymore), thus making the bass wider and often more clear.
Blending in as little as 10% of the wet effect can often made the bass pop out in the mix and be heard. There's a reason why the bass on Roland's legendary Juno sounds so good - and it's that famous chorus effect. The bass absolutely surrounds you!
Another great way to widen your bass (which yes, you are allowed to do!) is to use a mid/side equalizer in tandem with an effect. I love to add a reverb or chorus to my bassline first, and THEN patch a mid/side EQ in just after that. Boosting the side channel with a broad stroke around 100-300hz can really bring out the stereo effect you just added. Often the first and third harmonics of the sub lie in this area, which sound pleasing to our ears when boosted. Try it! It doesn't work everywhere, but when it does, it is a wonderful trick. Happy composing!