what focal length and aperture are you using?
although diffraction starts to be significant with crop-frame cameras beyond f11, the extra depth of field at f16 may offset the loss of center sharpness - generally wouldn't go beyond f16 though - even though you can preview the depth of field in liveview, i generally shoot flowers at 2 or 3 different apertures as i'm not experienced enough to visualize from the camera to a print
my experience with the 500D is screw it in and use it - have always had excellent results, and often prefer it to using extension tubes
if you had to shoot this at 200mm for magnification, another alternative would be to add an extension tube to shoot at a shorter focal length