I haven’t built any robot yet, but lots of other RC controlled stuff. Generally, small DC Motors have no problem with voltage somewhat higher or lower than rated. Even 50% or more are ok, as long as you keep an eye on the amps. Generally, you should never exceed half the rated stall current during normal operation. For prolonged periods, 25% of stall current or less is what you should aim for, because there you get decent efficiency and not too much heat.
However, with 7.4 instead of 6 V I wouldn’t worry about anything.