Of course you CAN do it without doing the crank, but I'm from school of if you're doing it, do it properly.
I'm a professionally trained engine builder and have spent a lot of time balancing cranks, you'd be massively surprised how much a modified flywheel throws a crank out of balance and a properly balanced rotating assembly makes a huge difference to the general 'happiness' of the engine, bearings live longer and friction is reduced, everything opperates more smoothly and quietly.
It's up to you, but I'd always do the full package if I was going to mess about with engines.