Once the override button in the Parameters tab is clicked, it won't be restore.

[version 1.5 and 1.8-develop]

Walkthrough:
1) Create a host-group with a parameter
2) Edit one of the hosts
3) Set its host-group to the one you've just created
4) Choose the Parameters tab
5) Click on "Override" button next to that parameter
6) Click on the "X remove"
7) If you change your mind and want to override that parameter, you cannot do that unless you refresh