I guess it is, I do it in preInit too. The important part is that you do it before adding any of your Recipes. Adding Recipes does not have to be done in postInit, it can be done as early as you like, obviously your Items should exist and be registered to the Item Registry when you add the Recipe though. 
One famous case of that being fucked up is Factorization, where the Acid Bottles get registered to the Ore Dictionary before they even got an Item ID assigned, causing CoFH-Core to crash. Luckily Asie fixed that one later on. ^^