I've never had a cat knock down a tree, but for those with more rambunctious felines, the wall anchors sound like a good idea.
There are other hazards to Christmas trees to be aware of, as well, but if you take a few precautions, you and your kitties can have a safe holiday.
I would suggest keeping the fragile ornaments towards the middle to the top of the tree in case you're cat decides they'd make good toys. Some cats also like to chew on electrical wires, which can be very dangerous, so, if you have a chewer, make sure wires are not hanging where they are easy for kitty to reach.
Tinsel is another thing that can be dangerous for your pet. Shiny, dangling tinsel...so tempting to play with! Tinsel, if eaten, can become tangled in the digestive track, which would result in emergency surgery to remove the obstruction. Better to skip it altogether I think.