I always put any wheel I've taken off for work on suspension, brakes etc under the the sill as a failsafe as have had a car topple off an axle stand many years ago.
I generally prefer a lump of something solid under the sill (wheel with some packers under the sill is a good call)...I have a big lump of rsj offcut for the job too...if I'm taking both front/rear wheels off something heavy like my XC70 then I'll use the rsj offcuts to be safe!
I also have a 10ton axle stand which is a screw type so you can adjust the height....that's great for just throwing under the car as a fail safe....wish I had bought 2 of them but they weren't a cheap option!
I have a pair of tractor axle stands, rated to 10 tons. Nice and sturdy old things, thread on the stem to get fine adjustment. unlike the flimsy modern ones. Also have a pair of halfords ones too, to be honest there not to bad either.
But peace of mind, nothing beats sleepers, or more recently Ive been using 9x3 timber I cut up, into blocks about 18 inches long.
Must by me a highlift trolley jack one of these days, as the normal one needs a block to use on the pickups.
Always use axle stands and wheel chocks. Any removed wheels are placed under the car as plan b and lastly the two trolley jacks on the sill jack points either side just off load bearing. It's served me well up to now, given i don't have a workshop with a lift or pit.
I think the key is to understand what you are supporting...if it's a mini which is totally stripped then not really that heavy. If it's a 2,000kg volvo then you're going to need to have a think!
My rule of thump is to look at the weight stamped on the car....divide by 4 and thats a ball park for what is going to hit you on the head when 1 wheel is off...if more than 1 wheel is coming off then I always think a bit harder!
Depending what you are doing governs my criteria
For example supporting a car for minor welding, not much consideration
Support in a car for major then spirit levelled so as not to twist or worse weld in twist, or even braced - welded in structures for apatures before cutting.
Often one end on ramps and other on stands allows flexibility if ramps are not in the way.
Just make sure the base is wider than the jack top. I've seen 3 point axle stands sink into tarmac over time admittedly a very long time. Wood is best for long term work and always have back up this is coming from some one who a dropped a car from relying on the weakest link of safety ME