My opinion is that the break in should be done with a hotter than usual plug (since the engine isn't pushed as hard and doesn't get quite as hot). The plug is more likely to foul if it doesn't reach it's self cleaning temperature and cannot burn off the carbon deposits.
My most recent experience with this is that I ran the same plug for a year and a half (hotter than stock), but unexpectedly broke a piston. A knowledgeable person noticed that I was running a hot plug and thought that it contributed to the demise of my top end. I switched to the stock recommend B8 plug, and it lasted 3 rides before fouling out. Simply put, I don't keep the engine screaming long enough for a B8 so a B7 suits my riding style. Take this story for what it's worth to you.
Of course, there are those that feel that I have no idea what I am talking about, so maybe they will step in and dispute this, if not then I am probably right.