When in doubt, refer back to this book by Denny Cherry. Here's an article talking about it too. Read #7. With an AD account, you can set the minimum possible security needed. It's more work, but it's the right way to go.
"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