Personally, as little as possible. Rates are low, and the political/economic situation is volatile. Let the bank hold the risk.
You’re better off with a shorter term... use small chunks of your savings to cover 2-3 payments over a period of years. That is my one regret —- my brother scrimped to afford a 15 year loan and will own his house in two years. (He’ll be 41) Run a few scenarios, changes to the deduction structure for property taxes impact the net in high tax states.