I would say best time to do it is when you have time. It's a pretty easy task to complete. If a EGR cooler goes it's puke coolant out the degas bottle much like a blown head gasket. Also if coolant gets into your intake it can cause the heads to blow because water Doesn't compress.
2007 F350 CCLB Lariat 4x4. EGR delete, 4" turbo back straight pipe 5'' tip, coolant filter/CAT ELC, blue spring mod, DC Power 185 AMP Alternator,HFCM plug Upgrade, Everything else STOCK. 173k- Feb 2017