Err on the side of something
Definition 1 of 1
Pro Tip 1/3Risk Aversion
The phrase suggests choosing a less risky option when unsure about a decision. 
Since we don't have all the facts, let's err on the side of caution.
Pro Tip 2/3Metaphorical Edge
Imagine standing at a crossroads; one side is safer. You 'err' towards safety. 
I'm not sure how much to tip, so I'll err on the side of generosity.
Pro Tip 3/3Subtle Uncertainty
Using 'err on the side of' suggests there's some doubt or uncertainty involved. 
They could be here by 8, but let's err on the side of caution and say 9.