Good day to everyone

I'd like to ask a question about the English short forms of pronouns and verbs.
I'm writing a short book about a technical topic, but use informal English (it's more like a tutorial).

Mostly I use short forms such as I'm / We're as I'd like to get closer to the reader,
but sometimes I think the long form looks or sounds better in specific cases.
What's the general opinion on this?

My main concern is about consistency as I once learned that after deciding for one form,
it should be used consistently without switching between different forms.