...are plasterboard screws worth trying?
Taking the question literally, then yes!
They have a trumpet-shaped head, flat at the edge, as they are designed to pull slightly
into plasterboard, but...
... to get a pocket hole joint to pull up properly you need the right tension, meaning in this case enough tension/torque for the purpose. That may be hard to do, as Phillips-head screws do tend to cam out.* I have a diamond Phillips bit -- they are easily found -- and that makes life much easier, but it wouldn't fit in a pocket hole, as the adaptor is too wide. You might find that a bit intended for an auto-feed screwdriver system would fit, but that might wipe out any saving from not using Robertson screws (the ones branded as Kreg).
I'm sure Jacob is right about countersunk screws, but the key thing he said is "... [no worse than] in any other situation.
." So let's consider that for a sec:
The obvious thing is to try it
- The as-drilled pocket hole has a squared-off shoulder, and the pilot hole part doesn't match the shank width of a normal screw (probably), it's narrower.
- If it's natural material, the first part of the wood in the joint is effectively end grain. If you sink a countersunk screw in end grain, with as little material surrounding it as you get with a typical pocket hole joint, I'd expect it to split some hardwoods. Man made boards probably wouldn't be as bad.
- Plasterboard screws are designed to hold in softwood. They have a very coarse thread, and wouldn't be suitable for some applications. Unlike the proper Robertson screws, I don't think you can get the right length in a range of threads.
- On that topic, the pocket hole system does need fairly specific lengths of screw. My Kreg system has a table of what you should use and I find 1 1/4" is the most common size I use. If you can get that in a plasterboard head..
, but with the same stock at roughly the same dimensions that you intend to use. And, if it is natural wood, put the finished test joint in a warm place (in the airing cupboard or on a radiator), as shrinkage encourages splitting - you want to find the worst case, before you find it when you dont want to.
PS: there are other advantages to Robertson screws, such as the fact they sit well on the driver (even a worn one), and have almost no tendency to cam out at all. I like them, and bulk prices on the internet aren't too bad really.
*You live and learn: apparently the idea that this is intentional is an urban myth. I used to believe it but, thankfully there are more knowledgeable people on the forum!