the "grains" point is a good one, though by my experience with other lines, I've found that a rod well matched to 15' 9 wt tips can usually cast and turnover 20' of LC13 just fine. But, you must slow down the casting stroke and let the butt section do the work.
On the other hand, if you expect to cast LC13 as your main tip most of the time, then by all means match the grains for that use.