I have no idea what that means. But I'm just playing an E, and I don't play chords, so there's no problem. And if I said something like that to a guitarist, he would be justified in thinking that I'm a complete ass.
I mean yeah if you phrased it like that, pretty ass-y. But that sort of thing, tastefully done, can really add another layer to a song. One of my bands has a kind of jazzy song (jazzy for that band at least) that is a lot of F C and G in different patterns, certain spots when the guitarist is playing C I'll play a C root, and in other spots he'll finger the same chord and I play a G on the 1 instead. Not as extreme as turning an Em into a Cmaj7, but it helps give the song a little more sense of direction, movement and tension. This band is somewhat experimental and down to do some weird stuff... Sometimes I play around with other intervals and usually get the stink eye, but have sold the BL on a few of them!
And yes, you should know if the guitarist is playing Em or Emaj, because if you want to expand your part out past the root note you need to know what other notes are in play. Is the third of the chord G or G#?