Nanine, I think the days of just dropping in when passing are long gone.
I would hestitate to do so, irrespective of whether it was near family or friends, although both groups know they are welcome to do so here now we have retired.
I think the best thing is to find a way of nicely asking your DIL what suits her best.
How you do so, depends greatly on what terms you are on with your DIL. I would probably myself say, "It used to drive me mad that my MIL never asked if it was convenient, she "just dropped in in passing" and never took a hint. That's why I more or less ring and make an appointment. Is this driving you mad?"
Most paternal grandmothers on gransnet feel they are treated differently to what the DIL's mother is. Some doubtless are, others may just "feel" this.
In my experience, we have to be careful not just to "feel" something or other in relation to our adult children, but try to find a way of asking about whatever it is.
Sez she, who for months has tried to find a tactful way of asking bonus daughter whether she and her husband are at odds. I "know" something is off in that marriage, but also know that while it worries me, it is NOT my business unless my advice is specifically asked for. Not the same as your problem, but definitely the kind of thing where you have to bite your tongue.
Whether you should ask, or bite your tongue here, I cannot possibly know. Often with family "least said, soonest mended" is still good advice.