I have a 1989 Ford F-150 pickup with about 350K miles. It needs valve cover gaskets, plugs, wires, muffler and other 'odd and ends'. Can I tell, from a compression check of the enging, whether it is worth spending the money on repairs? How hard is it to replace valve cover gaskets? I am an amateur.

Yes, it will give you a pretty good idea of the engine's condition--good, basic, fundamental data on the valves and rings. Replacing valve cover gaskets are easy to do. Most of the time will be spent cleaning sealing surfaces.

Compression should be fairly even on all cylinders. One that is really off indicates a bad cylinder. $$$
If you changed the oil regularly it would help prolong the life of your engine.
Valve cover gaskets are a few bolts and easy to get to--right on top of the engine. You have to make sure the surfaces are scraped clean before you put the new gaskets on, and use a little "perma-tex" when you put them on.
If you can get it in your area, try something called "Power Punch" Use 1/2 a quart at oil change (replacing the same amount of oil). It's great stuf, especially for an older engine.