As a rule means usually, though not always. As a rule I walk, unless it is pouring.
It describes a habit rather than a regulation, which is the trap in the word rule. It is nearly always followed by an exception.
1phraseas a ruleUsually, though not always.
“As a rule the shop is shut on Mondays.”
as a rule is a phrase.
Practised in Small Talk & Everyday Phrases