Can you provide some sentences wherein you're not sure which tense to use?
As a general rule, use Past Simple when referring to the present and future simple — to the future. But since the former extends naturally into the latter, these tenses are sometimes interchaneable.
"Lock the door so that nobody can bother us."
"Lock the door so that nobody bothers us."
"Lock the door so that nobody will bother us."
"I have re-bound the book so that it opens flat easily."
"I have re-bound the book so that it won't fall apart when opened flat"
This is only my humble opinion that may need serious correction.
EDIT:Are they both ok?:
1. "Enable coockies so that you don't have to log in every time",
2. "Enable coockies so that you won't have to log in every time"
Looks like it makes a difference whether it's a clause of purpose or of consequence...