WD40 leaves an oily residue which over a year evaporates to a sticky mess. I would keep it away from open switches for this reason.
I thoroughly recommend a cheaper alternative called GT85. This is much lighter, a better cleaner, and leaves only a slight oily residue which is claimed to contain PTFE.
http://www.gt85.co.uk/about.htm.
Only trouble is the smell is stronger than WD40!