How many guys strap their boats down to the trailer when towing ?

always.

I never have, but it wouldnt be a bad idea

95% of this time YES
Been known not too at times but only locally towing
But then again mine is 28' with a pair of 502's in the back. I do not worry about it bouncing, but more about it sliding back. I have the bow strap on along with the support bar, but still worry when the back is not strapped

Yes but, I just snug them down. Sometimes I will trailer the boat a couple miles to let it settle on the trailer before I snug them down. Depends on how the boat sat on trailer when I pull it out of the water.

The front strap, 2" at least, is the real important one. In a head on collision, you dont want the boat killing you. The winch mount/front v wont stop it in most cases.

Always.
Last year after a road trip and where I had to hit the brakes, I noticed my boat was a bit further (1-2") than normal on the trailer. Thought it was odd, until I seen fresh scratches on the trailer were the wench stand had slide forward. Didn't think I hit the brakes THAT hard, but chit moved!
32' boat with twin 502's. That was even with both stern straps, 3" winch strap and 2 bow straps to trailer. There's alot of momentum back there. Like MnFB, it's not the bouncing, it's the sliding that I am more conscerned about.

A couple years ago I was driving behind a 33' Daytona on the trailer when it hit a bump...it came up a few inches or so off the bunks, and landed pretty hard.

Now I always strap mine down.

Always. We blew a front tire on the truck a few years back and ran off the road. The boat was strapped down but still slid forward almost a foot. It tore a strap in half on the back and moved the winch post forward.

Always. Better safe than sorry.