I usually use a rule of thumb: stock ignition system, stock gap; higher energy system aftermarket, wider gap. How wide, is often stated by system manufacturer, if not, I'll add 5 thou. I just did a Pertronics II dizzy with the Pertroinics II coil and some of the literature stated to use stock gaps, other info implied gap may be increased but didn't say to what. I called Pertronics and the guy said I "should" be able to increase it but was reluctant to commit to a figure. Finally he broke down and said some users have reported good results with up to 10 thou over stock. I went with 5 and all is well. However, if going with wider gaps, make sure the plug leads are able to handle it.
Unless in an emergency, I can't image installing plugs without checking the gaps first.
