WD-40 is marketed as a de-grease, Water Dispersant and anti-rust coating and makes a good general cleaner, but never use WD-40 to lubricate anything mechanical as (from bad experience) it tends to set like soft varnish after a while and actually attracts lots of dirt - a good silicone spray (WD-40 people make one) is a much better lubricant. Do not use silicon on your EGR valve however because it will damage the oxygen sensors (remember the dodgy supermarket fuel episode about 5 years ago where silicone in the fuel wrecked the oxygen sensor on users cars).
I certainly wouldn't use WD-40 where high temperature is involved (eg, on an EGR valve) because it will do a lot more harm than good. While white spirit may remover fuel laquer build up, kerosene or paraffin will do it a lot better and non of them to my knowledge remove carbon deposits - which is exactly what causes the EGR to stick.
I would buy a proprietry carburettor cleaner and a little wire brush from halfords etc. -turn valve upside down and soak the mechanism in carb cleaner for a while to loosen the deposits and be prepared to do a bit of mechanical cleaning and thumping as well.
An EGR valve stuck open will be like an inlet vacuum leak and will cause hard starting and jerky low down running. A valve stuck closed will just cause higher NOx emissions at higher revs, and may cause higher fuel consumption. If you want to prove that your low-down running problems are cause by EGR valve you can blank it off by slipping in a sheet steel gasket without a hole to blank off the passage completely, It is not a good idea to leave the gasket in place for too long though, as it may cause other problems.