No spread (decent) would ever think of suggesting changing the direction you work lol.
and yet most have aches and pains. I wonder if there's a connection? Could it be the years of making exactly the same arm movements? If only there were a name for that. Oh wait, there is. Repetitive Strain Injury (RSI).
Most trades will trot out the 'only one way/direction to work' rubbish, when what they actually mean is "that's the only way I've ever done it". Right handed spreads work one way, left hand another, but that's their dominant hand, not the only one. Anyone can teach themselves to use the 'other' hand for any task that they can do with their dominant hand, it just takes time and a bit of effort. That effort to get your brain used to it will provide a mental challenge akin to learning a new skill. The physical change will develop muscles and muscle memory which will take the load off overused muscles and balance out between the two sides of your body. Your core, hip, shoulder, chest, fore arm, wrist, neck even your legs are involved in a trowelling action and just putting the left and right sides out of balance will cause problems, never mind the strains that are associated with doing the same thing day in and day out for years.
"Change nothing, nothing changes."