Here is a list of
bolt grade markings and their strengths.
You should not automatically increase bolt strength grades. Certain applications require certain properties. A grade 2 bolt will have greater stretch before failing than a grade 5 and provide a cushion of sorts when forces exceed it's strength. I can give an example of this. Back in the 90s some school buses had the seats attached with grade 8 bolts. Studys found that during a crash these bolts would fail suddenly and cause greater injury. A recall was issued to replace all the seat bolts with grade 5. The grade 5 bolts while not having the same tensile strength would stretch and create some cushion when they failed.
Some PTO driven implements that use a shear bolt will require a grade 2 bolt to protect the gear boxes. People often replace those with grade 5 or 8 bolts to reduce the frequency of failure but they do so at the risk of damaging the gear box or PTO components.