It is an old chestnut - worth using the search function for past discussions.
At the end of the day it all depends on the owners, whatever the flight guides might or might not say - so never assume unless you are sure of your alternate or experience tells you otherwise.
In the UK for example Duxford are hopeless. They are strictly PPR by 'phone only. They are a good example of the nonesense of sticky rigidly to PPR by telephone whatever the circumstances (and of course it is their privilege to do so). I was passing them a while ago one winters day, with no activity in their "ATZ" at all, and all of about a lone aircraft on the ground.
Could I pop in for a coffee I asked.
Did you 'phone for PPR?
No, said I, as I was passing I thought I might just ask.
Then you cant, said he.
If I had 'phoned, might I then have been allowed.
Yes, said he, we have nothing going on at all today.
Ah, said I, so if I land at Cambridge in 10 minutes, and call you on the mobile for PPR, all will be well.
Ah, yes said he.
Well what about if I decend to 1,000 feet in your overhead so I can get a mobile signal and call you from there.
At this point the conversation was getting a bit strained and I think he thought I was taking the Mick so we carried on about our business.
Bembridge on the other hand comes to mind. They say they are PPR, but are always welcoming, never had a problem give them a call in bound - a great place to visit.
To be fair you would be nuts going somewhere that was PPR without checking without an alternate, but sometimes I think there is nothing better than setting off on a round robin and deciding on the hoof to call into somewhere en route. As long as you do so on the basis that if they turn you away it is only their loss, not yours, I dont see that it matters asking.
If it is a genuinely a private strip (personally I dont include places such as Duxford in this category) then I think it is just rude not to 'phone first unless you know the owners already.