Yep, necessary.

Cant remember how often. I do it when I am messin with stuff or if I think they are getting too noisy.

suzuki service manual :
valve clearance  (.003-.005 in) @ 600,4000,7500,11,000,15,000 miles
"before adjusting the valve clearance,check or adjust the automatic de-compression cable play."
de-comp play  (.12-.20 in)  from the body with piston @ t.d.c. on the compression stroke.