I don't think this will have a major effect since McDonnell also has a high rating. The reason for Deeds getting the endorsement is that McDonnell supported one-gun-a-month back in the day so he has an A- rating, while Deeds has an A (its funny since now Deeds says its a good idea and McDonnell now opposes it).
In fact, it could prove bad since Deeds needs NOVA to win this election, and this will probably hurt his chances there.
The is no question as to who the NRA will endorse in the Governor and Lt. Gov elections. Jerry Kilgore and Bill Bolling are guaranteed to get it.