In a longer barreled 1:10 twist bolt gun I've seen good results with Match Kings up to 180 grains.  Too much beyond that and you get to a point of diminishing returns.  The 180 grainers are only really necessary if you are trying to stretch your range out beyond 600 yards, since they shrug off the wind a little better.  Usually out to 300 yards plus, the 180 grainers don't do much that the 168 won't do.  Of course every rifle is different so buy a box of 20, load them up carefully, and try it!