i have always been outspoken FOR open feeding so there is a bit of Bias there.. Hehe, sorry pistolpete!
Open feeding this time of year shouldn't cause any problems. I enjoy it a lot. Once it comes time for neighbors to put supers on its a different story.
I doubt bees even consider the "nectar" source as flower or jar, they only know what they get from it. I have not tested different distances, but have not fed closer than about 75 yards. Last summers dearth had robbing in progress on several hives, it was my first real knowledge that the local sources were drying up. I open fed, and within an hour the robbing had mostly stopped. I also added screens and blocks, so I am sure that was part of the deterrent.
Bumble bees, wasps, orchard bees and other bees I cannot name came to those feeders, Probably quite a few of my neighbors bees as well. All were welcome, except the yellow jackets. My trusty stick smashed them as fast as they landed where I could reach them.