There is no way you could have much effect on the tip of a valve stem by attacking it with a hand file. The stem is nearly as hard as the file and much, much tougher. You could do the job with a hand-held diamond lap, or with an oil stone, but I think a bench grinder is quicker and will do a better job. There are two key considerations.
First, the tip of the stem has to be square to the axis of the stem.
Second, you must not allow the stem to heat up.
The easiest way to do the job, though don't expect a TAFE instructor to like it, is to clamp a short piece of steel about 10 mm square with a straight edge, to the grinder rest, so the edge is exactly square to the side of the wheel. Then slide the valve stem along the edge of the piece of steel so that it bears lightly against the side of the wheel. Using the side of the wheel is strictly forbidden of course, not for safety reasons but because you can only groove it once, then you can't dress it without wasting a whole lot of wheel. However for this type of job I personally do it that way. Remember, only light brief contact between stem-tip and wheel, then stop, long before the stem starts to heat up. Remove any trace of a burr from the stem tip with an oilstone. Then wipe the stem to make completely sure there is no abrasive on it (from the grinding wheel) and try the valve in the engine, just by dropping the valve into the guide, holding the valve head down with your fingers, and checking the tappet clearance with a feeler gauge. Don't bother to assemble the spring and retainer, it wouldn't help, it would just waste time. Then if the clearance is still insufficient, go back to the grinder and touch the wheel lightly for another couple of seconds. Repeat as many times as necessary to get just the right tappet clearance.
Remember, you need to inspect the valve seat inserts to check that they have not been moving in the cylinder casting.
