A careful flashing test is important whether the flashing is non-localized, localized, masked, etc, but particularly if it is non-localized (ie the entire sheet of paper). Flashing WILL decrease local contrast - and that effect will extend downward further into the midtones and darker values than most people assume it will. It is often helpful in the initial flashing test, to back off slightly from the maximum exposure that appears to generate no tone. This depends on the type of highlight one is attempting to bring in, and also on the overall tonality of the image (ie is it mostly dark tones with some tricky highlights, or is there a substantial light area etc).