its really preference, i hate short skis, they are really squirlly, not the bigggest landing platform, some kids will tell you they are easier to spin, w/e just grow a sack
is that 4 cm because if i mount 168s 4 back there wont be a whole lot of tail im assuming. I dunno im so torn between sizes. arggggg. When I'm not skiing the park I usually do tree glade runs if that helps at all
i had my old 168cm scratches mounted at the modern line(7cm back from true center). i wanted more tail so decided to go with 174s for my new set and i mounted them -2 from true center. i havent noticed anything negative about doing so. the skis dont feel longer when riding normally(although they are a bit wider than my old set), swing wieght hasnt changed and i would actually say the further up mounting makes spinning easier, and switch landing and riding is definitly easier than before.
i dont think making the jump from 160 to 174 will be too hard to adjust to. and if your still growing that will give you some ski to grow into as well.