I've come to form the opinion that there is almost no need for the word so. In narration anyway.
Why? Well I'll tell you!
SO makes for wordy complicated sentences. Also, it's explain-y.
He knew he was in trouble so he hid in the bathroom so the teacher wouldn't see him.
Um... yeah. See? Wordy. Long. Complicated. Telling. You could say all this much more actively and with less explanation.
Footsteps in the hall. This was it, if the teacher saw him he was busted. He jumped into the nearest doorway, the girl's bathroom, just as she rounded the corner.
SO frequently states the obvious. It strings together two sentences that are often better left apart.
It was raining out so I grabbed my umbrella on my way out the door.
Since we were just told that it's raining, we KNOW WHY she grabs the umbrella. So makes the sentence almost feel condescending. And it would be better said in two short sentences.
Rain pounded on the roof. I grabbed my umbrella on my way out the door.
When you juxtapose two things together, (it's raining, grabbed umbrella) readers make the connection on their own. It doesn't need to be spelled out.
SO is lazy. It's like "and then".
Chances are good that if you're using so somewhere other than dialogue (where it can make for lovely, awkward conversation), you're telling. Which is lazy. Also, whatever you're telling about is probably boring and unnecessary.
He was tired so he went upstairs and took a shower so he wouldn't smell like french fries when he woke up.
This is just a list of things he did with explanations about why. It's not particularly interesting and the purpose of it is really just to say that he went to bed so that we can move on to the next scene where something actually happens. Boring and unnecessary.
I was hungry, so I went downstairs to get a snack. Then I was attacked by a ninja!
Is it really necessary for us to know why he went downstairs? Isn't being attacked by a ninja the important part here? In this case the so sentence is being used as set up and that's exactly what it feels like. Like we're being lead to something rather than experiencing it as it happens.
I was staring into the fridge trying to find something that wasn't expired when out of nowhere, ninja attack!
So... the next time you find yourself using SO stop and ask yourself, is it really necessary here? Am I telling? Can I say this more succinctly? Can I say this more actively? I bet you'll find a better way to get your point across!