That's one of those "conventional wisdom" tenets from way back. Sometimes it works, sometimes it partially works, but that's based only on my cursory glances over years of occasionally trying the procedure. I've never used my borescope to verify how much lead was actually removed or how much may have been ironed into the grooves.

When I have leading, I prefer a brush and solvent. Ideally, however, it's best to figure out what is causing the leading and correct the problem.