We put them under the tree ahead of time. When Santa comes, he puts the special/big gift out, pre assembled if necessary, and usually unwrapped.
But we usually go away to visit family on Christmas, so many presents end up wrapped and put in bags or boxes, ready to load into the car. We have a few presents out under the tree that won't be going with us and the kids have not really ventured over to peek/play with them other than to point and say that they want to open them. They are 4 (twins).
When we arrive at our destination, we put all the presents out. There are a lot of them scattered around because there are lots of relatives there.
When I was younger, I remember shaking a few presents ahead of time. If my mom caught me, she'd tell me to stop. :-) When I got to be around 11 or so, sometimes my mom would put gifts for my sister and myself in plain boxes and ask us to wrap them. We never looked. At some point along the way we learned that peeking just ruined the surprise.
Unless it's breakable, I don't think I'd mind if my kids shook the presents. At this age they probably wouldn't guess what they are anyway. When they get older, I'm sure they will be able to guess some, but I also try to get a little creative with gifts that have an obvious shape by putting them in an unexpected box size. Or you can put some pebbles in the box so it makes a noise they aren't expecting. In my family, you can't always believe the box. So just because the box shows a blender on the outside, it may not really be a blender.