afish - I very rarely fish privately owned land. I’m a small stream guy and there are more small streams available on publicly owned land (State/National Forest, SGL, State Parks, Nature Preserves - check the specific rules individually on those) in PA than I’ll ever fish in my lifetime. I generally just choose to avoid privately owned land, posted or not, and don’t worry about it.
The only private land I regularly fish is in the upper Kettle watershed, near the camp I’m a member at. I know the neighboring camps and have permission to fish there, though none of it is posted, and in that area folks generally allow walk in fishing without asking.
To answer your question, no. I would not fish private land, posted or not, without knowing for certain I had permission to be there...Either direct from the landowner, or through obvious fishing permitted signs at the location. I wouldn’t make a drive to a stream without knowing for certain first, or a back up plan in place if I couldn’t confirm permission to fish on arrival.
Are there some streams I’d like to fish but have written off because they’re on private (unposted in many cases) land, and I’m not certain of the landowner’s wishes? Yeah. But I know plenty of good ones where I know for certain I’m welcome to fish there. Could I fish some of those unposted streams? Yeah, probably, but I’d rather not being worrying about stuff like that when out fishing.
Edit: I’ll add that none of the above makes me right or wrong in any way. It’s just how I choose to handle it and spend my relatively little valuable time to fish. As long as what you’re doing is legal, I’ll have no issue with someone who chooses to look at the above differently than me.