For cropping, you may find you don't need to do any fiddling - it's dependent on the GI in your scene. If there's no GI, there shouldn't be a problem. If there is, you may find adjacent crops are lit slightly differently, leading to a visible seam. One partial remedy for this is to increase GI blurring, from its default setting of 8 to anything up to 100. If that doesn't work, another aid is in the Advanced tab of the renderer. You want to choose for Ray detail region, "Detail in crop region", EDIT - PG says "Detail in camera" - I thought that was for stitching whole renders together, not crops, but perhaps someone from PS could confirm? and then directly below that, Ray detail region padding = 1 (at least). Also, you should try to have at least 5% overlap between crops, e.g. 0-0.25, 0.2-0.45 etc. Hope that helps!