This greatly depends on the size of the trees you will have on the stands and the amount of wind exposure.
If you have larger trees, say between 25-150+ lbs, stands planted in the ground are a very VERY good idea. I have larger trees, most over 35 lbs some as large as 150. I put in four "monkey pole" stands for a few of them years ago. I used 4"x4" deck support posts that were about seven feet long. Buried the lower 3 1/2 feet or so in the ground, with the rest above. I constructed platforms from 1 1/4"x 1 1/4" deck rail pickets, as well as cross support underneath them attached to the upright post. The platforms were planned to be eight inches wider and deeper to accommodate my biggest bonsai's pot.
I don't use cement in the hole. Two schools of thought on that--concrete can hold water against the wood and rot wooden posts more quickly, posts without cement have to be buried deeper. Also cement in the hole makes replacing posts a gigantic pain in the ass, as you have to dig up 200 lb cement blocks along with the post...
A couple more things...
I have seen disasters with posts that simply sit on the ground. I won't use them for anything.
Wind is a big factor in using free standing posts. A big wind can cause heavy heavy drag on tree foliage, even on smaller trees. A good thunderstorm can mean big problems for posts that aren't sunk into the ground. Even if the tree is strapped down tight to the platform, wind can drag the entire thing over.
If you sink the pole into the ground, make sure you've dug deep enough. Nothing like having a post topple over a week later as the weight of the tree on the platform slowly pulls the pole to one side.