Valve clearances should always be checked even if they are deemed to be OK because you cannot hear anything and the engine is starting OK the clearance could of closed up which will then mean that valves will start to burn out and the engine efficiency will most certainly of dropped. Even a clearance which is half its optimum setting value will have a large effect on the engines performance but may have no real effect on the engines starting efficiency.
Barney your statement about most mechanics not checking valve clearances at scheduled intervals, is this based upon your own work ethics?
Adjustment of none overhead cam engines is as said the easiest usually a nut and bolt arrangement or a tapered bolt.
Overhead camshaft engines shim under bucket type, mostly camshaft out and can prove to be the biggest problem for the less skilled home mechanic so best left to a skilled mechanic. There are some which have an adjuster fitted which is a bolt with a tapered flat running its length which allows adjustment without camshaft removal, but I cannot recall this arrangement being used on any bike engines I have worked on.
Overhead camshaft shim over bucket, reason for this arrangement is so that the camshaft does not require removal from the engine. A specialised tool will be available for depressing the valve with the engine set into a certain point of its rotation so that the valve does not touch the piston crown when this operation is carried out, it prevents the valve head from being bent.
All shims should be changed for the correct sized replacement, **not ground down, **the shims are hardened to reduce the wear rate, remove this hardening and you might as well use mild steel shims.
LearnerRider, if you read the section in your manual and understand the procedure then you have a good chance of doing the job as long as you take your time, as you remove parts lay them out in order and where possible place bolts into their respective holes. New shims you will have to purchase once you know the correct replacement sizes you need (most bike workshops will sell their used shims separately for £1 - £2 a time much cheaper than new). Keep everything clean, and replace gaskets with new.
If you have any doubts leave it to the mechanic, or enrol on a home mechanics course at a college.
Best of luck.
assumption is the mother of all f*** ups.