Luck is one of the factors that the developers have hidden much (maybe due to the fact that it doesn't do much
)
But things it openly supports is critical shots, the chance for critical shots are dependent on luck and various perks.
Other things luck will affect is the chance for critical failurs while using weapons (or the people around you) and other minor stuff (such as special encounters - there is one where if you are unlucky you will find a parahia dog wich will cause bad things to happen
).
I remember vividly once having a char with low luck and I found that he often dropped his weapon or lost ammunition and such (as did party - annoying when you see peopel fumble for ammuntion all the time
).
Usually I keep my luck factor around 6 - but if I was to play it again I would go for 10 just to see
But as with anything else it is personal preference and you should play as you wish. If it is me, I would prefere luck more but others would not.