I guess I’m in the minority here. If I’m standing surveying the opposite shelves at the grocery or bookstore or wherever, I’m right up against the near shelves specifically to leave room for people to go by in front of me, so I don’t expect them to say “excuse me”. You could plausibly argue that I should apologize to passersby since I’m blocking half the aisle. I’m guess I’m inconsistent because sometimes in the reverse situation I will murmur it anyway as I go by.
I’m a big tall guy, and that intimidates some people. I compensate by being very polite. Yes, I say “Excuse me,” if I scoot between a shopper and her prey. I’ll stand and wait, if two carts are blocking the aisle. I’ll even turn around and go back, to avoid a jam-up. I occasionally help a short shopper reach something.
If we’re not sure which way to dodge the other cart, I laugh and say, “ah, the dance of the aisles!”
There are some items I can’t seem to find on the first pass. That’s embarrassing.
If I have to walk between someone and the products they are perusing, I always say “excuse me”. It’s not like it takes that much time or effort to do it, and it just seems polite.
Grocery store aisles should work like a highway IMHO. That means you drive your cart down the aisle as you would your car on a street. I loathe those people that drive down the middle of the aisle and worse, stop in the middle to browse the shelves. The absolute worst is when a family of let’s say four walks down the aisle next to each other blocking almost the entire aisle instead of the more polite single file way.
I’ll say excuse me the first time very politely to get them to move out of the way, but if that doesn’t work, the second time I say MOVE! I generally know what I want at the store and I always try avoid busy times at the store. So when I’m there, I wanna get out as fast as I can.
I always stay to the right, but I don’t signal turns. I accept another’s right of way at the end of an aisle, though. 
I always say excuse me. I wish everyone did the same. It’s just a nicer world when people are polite and friendly.