Use the sealant! It beats starting up the car and finding out the Water Pump is leaking.
When I was installing a new waterpump the gasket for the upper plenum connection wouldn't stay put so I just used a butt load of RTV and it hasn't leaked in a few years. Same with the left rear coolant passage, my headgasket kit didn't even have a new gasket for it, so RTV did the job!
Also, make sure there is no water in the oil. If you remove the heads without draining the block, Anti freeze will fill up the cylinders and drain through to the sump. I ended up driving 20 minutes with water in the sump and the engine has been rough ever since.