The 34 watt (4') T-12lamps were designed for use with 40 watt ballasts.
You will not experience problems in a genuine fixture (I'm not sure about the homeowner shoplites, which sometimes are meant to be used with even a 25 watt lamp).
If there were problems, the whole State of California would be dark. We haven't been allowed to use 40 watt lamps for years in commercial applications. To my knowledge,none of the manufacturers even make a 34 watt ballast.
Another case of this is that the 8' T12 Slimline lamp has been reduced from 75 watts to 60, also with the only result being reduced light output and energy savings...S
