I top crop Wyeast 1318 all the time, it is my house strain. I have kept this strain going from batch to batch for 10 different batches of beer before I opt for a new pitch. How much can you top crop you ask? A lot. I would not crop all the krausen, you need that yeast to finish the beer currently in fermenter. Generally speaking, if you crop approximately 1 cup of krausen and use that for your next batch you will PLENTY of healthy yeast and you should get a quick start on the next batch. I like to boil water, cool it, then put the top cropped yeast in the water in a flask and refrigerate it until I brew the next time. If top cropping becomes a practice you use a lot, I recommend two things, one use some servomyces or yeast nutrient when you brew and use pure oxygen for oxygenation of the batch. Repitching yeast that is chronically under aerated (oxygenated) and depleting nutrients can cause poor fermentations.
Please post back and let me know what you think when you top crop. The 1318 LIVES on top of the beer, you should have no problems.