When running games other than D&D, almost never.
When running or playing D&D... sometimes. And when we do, it is decent odds we are not worrying about every 5' square. When the situation is very simple, in terms of tactical positioning, I'm probably not using a grid, and use theater of the mind instead. When the tactical situation is a tad more complicated, such that it would be a burden and error-prone to expect everyone to remember all the elements in play, then a grid comes out.
When running or playing D&D... sometimes. And when we do, it is decent odds we are not worrying about every 5' square. When the situation is very simple, in terms of tactical positioning, I'm probably not using a grid, and use theater of the mind instead. When the tactical situation is a tad more complicated, such that it would be a burden and error-prone to expect everyone to remember all the elements in play, then a grid comes out.