Timid actually is one of the natures suggested for Jirachi on Smogon, ever go there, Mr. Brightside?
Just a matter of preference, but that's actually a good nature to use if you have a moveset with some special moves and a support move or two, while not having any physical moves.
'got Jirachi on D/P/Pt/HG/SS today, all five of which are Timid'