There are many more signs such as the roof leaking, excessive creaking at night(this is damaging your building) or through weather changes etc however the first mentioned are the early warning signs.
It can be a serious expense re-stumping your house however not near as expensive as it will get if left unattended. Those sticking doors and windows are under a lot of strain and are receiving damage as the situation gets worse. It's hard to notice the deterioration as it happens over a long period of time.
There's two things happening constantly with timber posts. The first is their sinking usually not all but the ones holding structural weight are the first to start. Secondly their rotting, as the outside more sappy part of the timber falls away with rot there becomes a gap, sometimes very noticeable sometimes not so obvious but it's there. This lessons the resistance around the post and narrowing it like a spear at the same time which over time gets worse and worse.
When the post subjected to the structural weight and pressures sink it puts pressure up through the roof and down to the posts in that area which puts excessive weight in the area leading to further problems.
Due to the amount of posts under a house it can take considerable time before the situation gets extremely damaging and to the point of unsafe however there will be no warning bar the creaking and aforementioned symptoms. At some point it will become unsafe in high wind areas and flood prone areas but there will be no warning at that point.
Safety or expensive damage? Mostly expensive damage, fixing doors windows and leaking roofs are expensive operations, getting to the timber joints that have been worked loose over years of the house crab walking downwards can be somewhat of a challenge. In addition to all this weather gets in and rots timber that needs to be replaced and so on and so forth.
If you act on the early signs often you may simply need to replace a few posts at a time and possibly over an extended period of time as keeping the floor level will slow down the deterioration to no end.