I hold the gate keys to about five of my small residential accounts, so I don't have to phone these to let me in.
But I do phone a small number of £35 plus accounts that I have (the biggest being a nice £90 job), to let them know I'm comming.
All I say is, 'Hi Mrs Smith*, I'm just tipping my hat to let you know I'm comming to clean your windows tomorrow'.
I've had no problems with this; unless they say they want the insides done too and they're currently in France (which has happened) and if they say they'll be out, I ask them to leave my payment under an object on their doorstep.
I wouldn't do it for a small account though.
*I don't call them all Mrs Smith; it depends on what their name is as to what I call them.