Using wonderlite cement board in my tub area surround, instructions say all edges supported? but almost every instruction video including their own doesn't show any backing behind the edges where the horizontal seams line up. Is this necessary or once the boards covered in thinset with FG mesh its as strong as the rest of the sheet?
If the framing is 16" on center or closer where the 2 long edges meet on the larger wall, I have never added any blocking. If the centers are 24", that is a bit of a stretch and needs to be blocked. I am presuming the narrow end walls are not an issue, since the sheet will go in vertically, and the seam will be above any reason to need blocking.
For tubs it is standard to install your cement/fiber board 1 vertical on each end (you can go out further but don't need to) and 2 horizontal on the back wall (and you can clip the top horizontal piece to match the ends but don't need to).
That being said the blocking between the horizontal pieces actually makes things a bit harder and in fact could cause problems. So first of all we are predrilling twice and using appropriate screws with probably a head less susceptible to getting stripped. The predrilling will be a very small bit for the screw and a wider bit to countersink the head. If you gets 2-3 screws in 3/4 of the way on a sheet you can do all of the predrilling in one go.
What does this have to do with blocking? Well it is normal practice for drywall to be installed with blocking and it usually makes it easier.
What is different about cement/fiber products? Well the alignment of the blocking (how flat it is and the fact that you are putting more emphasis on the "middle" section allows this very stiff product to run into a few issues:
The screws push in and the rest of the sheet is slightly veering out. I see this all the time with my new guys. I look at their shower/tub prep and get behind the framing and see it tight on one side and a 1/16-1/8" gap between the board and framing on the other side. Drywall bends to counter this. Cement board does to, to an extent, but not the same. With tile level, plumb, whatever... really doesn't matter. Just needs to be flat. One sheet going in at an angle and another sheet not following it is a bad thing.
I know it isn't always the case and you can do it many ways but when I hear blocking I think of two sheets meeting at a blocking line (we call them crossers by me). Well the problem here, especially for someone less skilled is that with cement/fiber boards it is easy to crack or ruin the integrity of the edge by sinking screws too close - especially on the corners and triple especially if point #1 is an issue.
Again know there can be many ways to do it but blocking almost implies the sheets right next to each other. I suggest that if two sheets of cement/fiber board are on the same wall they have at least a 1/4" gap. This is so your thinset can bind with the fiber tape without being on top of each board. If your sheets are butted you will have to thinset the top of the tape which produces a bubble that will make the whole install harder - especially with larger tiles.
Framing a shower/tub area surround right is crucial but it is also easy. Everything is at 16" or less and anywhere that has a rod or door that may hit it will usually be tripled. I will install blocking around plumbing or at cove heights or grab bard heights to future proof. But this blocking will for the most parts be set back 1/16-1/8" and will not be used to carry the boards unless I really take the time to make sure that the boards are perfectly aligned with everything else or they are part of a cove/nook.
Summary: There is not reason for blocking to support the weight given if you framed right so the potential (even if it is small) of it causing install issues is unjustified. Basically we used to do it and had issues. No longer do it (last 10 years), no issues.