They are more likely to come down with laminitis when they are overweight and underexercised, and when the sugars are rising in the grass. But it can strike any time and without warning
Personally I wouldn't be too worried about limiting access to
rough grazing, if you have it, over winter, the grass will have little nutritive value and won't be growing. You could probably leave them out 24x7 if the ground will stand it - we do ours unless there's some reason to have them stabled at night.
Come spring when the grass starts to grow, then yes you have to be careful. With my Fells, if I am worried about them getting too much I let them graze overnight when the sugars aren't rising and bring them in first thing before the sun warms the ground up.
In my view they're better with a good area of very poor grazing than being strip-grazed on better ground. If they are roaming 10 acres to find their forage then they are getting a fair bit of exercise just grazing, whereas you see a lot of fat ponies with hardly enough grass to walk around on - keeps the input down but doesn't give them the exercise they must have in order to avoid laminitis. I think Rosemary has hers on grass tracks around the steading - limited grass and they have to do some mileage in order to graze it.
If you only have better ground, then it's either graze it hard with your cattle and sheep before the ponies get on it, or limit their access to it. And yes, if it's good ground, you probably need to limit input for miniature Shetlands over winter, too - although I don't have Shetlands myself but friends who do find they get fat on air.