It's best to sight-in with a very steady rest.
It is not necessary, however. I have many firearms that were sighted in off-hand.
Elbows = bad.
Rests or proper off-hand technique = okay.
Just this month, I sighted in a 9mm pistol with a new red dot sight, for my father.
I didn't bother sitting at a bench, propping myself against a post, etc. I just started taking off-hand pot shots at small rocks or discarded shot shells and making note of point of aim (POA) when the trigger broke, and the resultant point of impact (POI).
Eight inches low and two left.
Six inches low and two left.
Nine inches low and three left.
Average that out, adjust, and try again.
With something fun and cheap to shoot (such as the 9mm with the red dot), I don't consider it a waste of time and ammunition. In some ways, it's good training - another way of making sure you're keeping your eyes open so you can make sure you know exactly where the sights/crosshairs were when the trigger broke - which allows a person to call their shots and makes downrange impacts that much more valuable to read (because you know exactly where the bullet should have gone - not just a rough idea of "well, I was somewhere in that bowling ball sized area").
For things that are expensive to shoot, hard on the shooter, or have a limited ammunition supply, it makes more sense to expedite the process. The following is also very EASY to do with scoped rifles.
I really like the concept of the "one shot zero" with solid rests and a solid bench.*
Fire a shot that you have confidence in being representative of the rifle's performance.
Move the crosshairs back to the POA. Hold the rifle dead still and have a friend** adjust the scope until the crosshairs are on the POI.
Fire a shot to verify, and the basic zero is set.
* There's no way to verify without firing a second shot. But, "one shot zero" the name that the concept is usually given.
** I do this alone. It's easier with an assistant, but I generally shoot alone. So, I've learned to hold a rifle dead still on my rests with one hand (normal rests, no cradles, sleds, clamps, etc.), while carefully using the other hand to adjust the scope.