I use single quotes when the string in question is used like an enumeration:

    set_color('black','white')

And double quotes for strings meant to be read by a human:

    print("Hello there.  How are you?")

And an example with both forms:

    syslog('warning',"unit %s spin rate %f too low",u,rate)

For me, it's an indication of how I expect the string to be used.