Purists will say no.


Practical people will say yes, of course.


There are two issues:

a) will it work (yes)

b) will it mar the headset (not if you are careful)


For b), if possible use a spanner with flat jaws, that fits tightly against the projections of the locknut. Carefully snug it down until it is tight enough not to slip, make sure the spanner is aligned with the locknut (I put use hand around it and feel with fingertips where the spanner grips the nut), and take it off. Especially if all you have is a vise-grip, use a pair of bits of thin ply, soft enough to crush/cushion/grip the locknut. Bits from the boxes satsumas come in are excellent.


Shimano using this type of locknut and cup configuration was the height of hubris[1]. I bought the special spanners when they were cheap, but before that I just used normal ones and care (and grease on the the threads - always. Always. ALWAYS.)


[1] internal cable routing is probably second, but lots frame makers have made that sin, only Shimano would use a birthday cake as a model for a bicycle part.