there is all sorts of horror stories of blockages id say having a back-up isnt a silly idea to put it into perspective-- lets asy it blocks your tank will overflow- loss of saltwater could cost a bit to replace if using ASW then any damage done by the spill, then you have your sump running dry depending on the design of your sump this may or may not affect your skimmer and heater but your return pump wont like running dry and may crap out. then say you are at work and your heater is in the sump its a cold day and no heated water is making it to the tank livestock could be affected. and depending how far along you are the cost could be in the thousands.
id spend the extra $30 to get another hole drilled and another $25 for extra plumbing just as a precaution