When it is an option, approximately what percentage of the time do you back into parking spaces as opposed to pulling straight in forward?

It takes me about 200% longer to back in than it does to go forward.

Maybe 2% of the time, it's easier to pull in forward.

I prefer to back in.   If you saw how people around here parked/drove, you'd back into a parking space 100% of the time.  They cannot seem to see any automobile backing out of a parking space - even if they've been cruising the lot for an hour waiting for an "up front" space and you're pulling out of one. 
Remember that friendly, frail great-great grandmother who told you all about her great-great grandchildren as you stood behind her in line inside the store?   Once she gets in her car, she will back out of her parking space at 35 mph, running over anyone or anything in her way... then give you the finger as she speeds off.  

Virtually never.  I have little to no ability to back into a parking space.  

I reverse perpendicular to the kerb when I need to load/unload a large quantity of stuff via the tailgate. As far as reverse parallel parking goes, I avoid that unless there's no other option. The biggest downside of station wagons is parking, but reverse parallel parking is the pits.

Backing up is not one of my greatest skills, even with a camera. If at all possible, I park where I can pull through and don't have to back in or out. We're all safer for it.

.00000001%

Also see Jane's response. 

Will pull in if possible. I also look for a space I can pull in and then go straight out.  I avoid backing up whenever I can. 

100%.  I have since I was 18 years old and went to work for Bell Telephone.  They stressed safety and not following the strict safety protocol when driving a company vehicle would land an employee in an uncomfortable place......maybe his own living room.    Every place you go to find the safest way to do things will tell you to always back into parking spaces.  People do not have wrecks nor hit pedestrians backing into parking spaces, but that  is not true when driving forward out of a parking space.  Some jurisdictions are requiring pull-through parking in lots and designing city streets with slanted curb parking the opposite direction with which you might be familiar....this demands backing into the spaces and pulling forward to leave the space.  Haven't you ever wondered why corporate vehicles always back into parking spaces and driveways?  

It’s almost 100% of the time.  When I’m looking for a parking space, I don’t consider how close it is to the store entrance.  I go for the ones I can pull through first.  If there aren’t any pull-through spaces, I back into one that has plenty of space.

I also do it this way to follow my own rules since I’m employed as the safety director for my husband’s trucking company.  In trucking, backing is always risky but backing into a space at the truck stop is far less dangerous than backing out of a space.  A truck pulling straight into the space at the truck stop is called nosediving and it’s a violation of our company policy.

If one of our drivers backs out of a space and hits another truck or a fixed object, it’s automatically a preventable accident.