Personally, unrestricted with lots and lots and lots of monitoring and alerts in place so that I can try to prevent the disks from filling up. Either approach has downsides. If you restrict the growth, you avoid filling the drive, but, you put a database into read only mode. Depending on the database in your system, that could be a resume generating event.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning