If you know what you are doing you can get an 18er down at most any level--some of the outfitters do. The trick is knowing what you are doing
The Rogue below 2k or so can get very technical. Sure the rapids are no more than class IV and there are only a couple that hard, but there are many, many shallow and tight class II to IIIs that could get you in trouble if you don't have good boat control. If you pack your 18 like a 16 you'll ride higher which will help offset the extra width and length. Take your time and watch out for Wildcat and Upper Black Bar--they cause problems all the time. Pull over and watch a few groups go through Blossom before giving it a go yourself. Have fun.