Some folks may have even seen a Clickjack Alert pop up when logging in to Blogger. Obviously, this doesn't provide us with a feeling of ease as we login. So, the question
Should I keep the "lock item" box checked, as NoScript recommends?is to be expected.
I think the decision to leave locked, or to unlock, any script that we use often should be made on two bases.
Obviously, unlocking any frequently used script, such as the Blogger login, is better convenience. If you trust any often used script, you'll want to unlock it, or end up verifying each time you use it. As long as there's no chance that you're being lured to an imposter web site (which leaves very little chance that you'll be logging in to Blogger), unlock the scripts that you run repeatedly, such as the Blogger login.
Unlocking any frequently used script is better security too. If you leave any frequently used script locked, you'll get used to clicking "Accept" over and over, routinely. One day, when you surf to a dodgy web site and are given the clickjack alert, you'll click "Accept" there too. If you intentionally enable trusted scripts, when you surf to a dodgy web site and get a clickjack alert, it will stand out in your mind and you'll be less likely to Accept a genuine clickjack exploit.
So unlocking frequently used scripts, even though NoScript may consider them potential clickjack exploits, is good for both convenience and security. If you trust the script, unlock it. And when you get a clickjack alert, don't accept it unless you explicitly know that the web site is trustable.