depends where it is, If it's in the car campgrounds or on the trails like spruce bog where the park maintains bird feeders and nesting boxes, I see little harm to it as those birds are socialized already.
If in the backcountry, definitely not, those birds are wild and feeding them at all is altering their natural behaviour.

The Grey Jays have usually taken my food before it occurs to me to feed them.

Interesting question. I am true believer that Leave No Trace means you need to take measures not to engage with animals. But, I've also hand fed birds on trails near our home. I think I fall into Scoutergriz' camp. I don't see a problem with it when doing it near established bird feeders (like the times I've done it on trails near home) but I would feel weird about it out in the backcountry.

Having a bird land on your hand is pretty special. If the birds already know to come to that area for food, I think it's a beautiful nature experience for young and old.