I've not noticed this on ny 175 motor, bu't worry too much about which valve mark is which. Just align the valve mark to the crankshaft pin and check that the valve is covering the inlet port when the piston on that side is at bottom dead centre. If not, simply turn the valve by 180 degrees. the valve needs to be closing the port at bdc to provide somevaccuum in the crankcase as the piston rises.
On an aside, the 350 motor seems to have this confusion built in, as it has only one mark on the valve. If you assemble a GTR otor "by the book", you'll end up with both valves pointing the same way - they need to be 180 prt to match the crank throws. Don't ask how I found this out - but it's REALLY annoying when it will only run on one cyclinder and you have to partially strip the motor again to fix the problem!
Graham