You'll have to evaluate it a bit - but basically - remembering the camera tries to average to to 18% grey - if there is a lot of dark foliage in the picture the camera will want to lighten it up a bit - this could be bad if there is a light coloured flowe in amongst it as it will get washed out - so that's when you so 'no I want it a bit darker' I like to take these in apperture priority so I can control the depth of field a bit more - but watch out for it selecting slower shutter speeds if you stop down too much
Have fun - experiment, I do!
Geoff Chandler. UK/England/Surrey
NIKON D90 / D80. Nikon 16 - 85 VR, Tamron 28-200,
Sigma 70-300APO, Tokina 100 AT-X Pro D.
SB600 flash. Panasonic DMC-TZ25