In SA the climate is generally low humidity and so you could leave the flour on the rollers, as long as you brew reasonably regularly.
In areas of high humidity and if you have mild steel rollers it would be better if you used a protector or spray the rollers after use with a moisture dispersant.
This would apply to Vlad and others who choose to "wet" their grain as it goes through the mill.
I had an email when I got my mill from Fred Francis at Crankandstein and he said as long as you use it regularly you wont have any probs.
The only ones he has seen that have deteriorated is ones that have not been used regularly and once again, only the mild steel ones.
When you consider all the facts, you would have to be using your mill on the beach to cause massive deteriation of the rollers and as others have said, if you lubricate the bushes/bearings and have a good method of driving the rollers without causing side loading on the shafts then any mill should give the home brewer many years of service.