Unless, perhaps, you are using animal manure in your worms bins as feed stock, I can not think of a reason to rid your castings of the aerobic microorganisms it contains.

If you sterilize the castings all you will have is dead castings. If they have been well worked by the wormies they will have little to no pathogens. I would guess that generic potting soil would be better if you sterilize the castings.

If you're looking for a mix to start/germinate tomato seeds, I would use a soilless seed starting mix, or maybe an equal mix of milled peat/perlite/vermiculilte. You want your seed starting mix to be sterile (to prevent the introduction of pathogens )and it does not need to provide any nutrients, because your little seedling will not require nutrients right away--it has all it needs in the seed itself.

Once my seedlings have two sets of leaves (the seed leaves or cotyledons plus one set of true leaves), I transplant them into larger containers with a richer potting soil. I usually mix in about 20% compost with a standard potting soil, add some extra vermiculite for drainage, and water with compost tea made from vermicompost.

Good luck with your tomato seedlings,
Emily in FRIGID Fairbanks

Worm castings in a seedling mix: Do the worm eggs in the castings hatch and eat the seedlings?