I think what some guys miss when washing is not necessarily their penis, but their taint, balls and ass.
@kherg007 has great tips for washing those areas.
Another issue that I've heard spoken about is Phimosis, when the foreskin is tight and doesn't ever retract.
Some SPs will not offer BBBJ for those with this condition because there is no way for the client to wash under his foreskin, meaning there is a buildup of god knows what under there.
I very clearly remember a member here stating that once he had a procedure and was able to finally retract his foreskin, it was the most god awful smell and gunk.