I was cleaning my E61 the other day, and after owning it for 8 years, thought I would get to the dispersion screw...
Tried to add to this old thread but it is closed
Cannot remove E61 dispersion screw
I used a fender washer with a crescent wrench similar to what Eric described, the screw came out with some careful pressure (10-14ft#?) and here are the before and after pics of the areas behind the screw. Warning: close ups make it very graphic just make sure the washer fits well and counter clockwise "looking up" force is closely controlled. Heating the head may help, but it was not necessary in this case. When reinstalled, a very small amount of silicone grease was added to the threads.
Surprisingly little accumulation after so long, but it feels good to get that toasted material out. I regularly water backflush and maybe once/twice a year detergent backflush clean, and once a year clean/lube the cam valves. Then every 3 years or so replace the brass plunger seals and springs (they will start to dribble and let you know)...Randy had a good writeup in this forum (I don't have the link) on the process and DaveC has a great flow shown at http://coffeetimex.wikidot.com/e61-group-servicing E61 maintenance really doesn't take long once it is experienced...Parts are inexpensive and readily available from Chriscoffee or Espressoparts.....
The dispersion area pics...Didn't post a pic of the cleaned up screw, but the back looked like the normally exposed area when the basket is removed...






Will it taste better? Of course