I recently received 2 large stoneware crocks with predrilled drainage holes in the bottom
and want to use them for planting in my patio area.
I live in Zone 6b (Madison CT) but occasionally get an extended cold spell.
One gardener says to fill the pots entirely with soil to prevent water freezing and cracking the bottom of the crocks and another gardener says to use a foam/filler in the bottom of the crocks to keep
the weight down and then fill 2/3 with soil. What do you suggest?

I have used many decorative pots in the past. I use potting soil mixed with topsoil. During the winter, I was on Long Island for a good many years, I would clean out the plant material at the end of the season.
I would cover the tops with wood boards in order to keep rain water from soaking the soil. In that way, no worries about the pot freezing and breaking.

If the pot is indeed stoneware (lithified clay) and it drains water, the medium inside can freeze solid without the pot breaking. It helps if the walls of the pot flare outward going up, so iced matter can move without stressing the pot walls.

I have left many pots outdoors through freezes with no problem. And if the soil in the pot is well drained, those little air pockets give the ice space to expand.

Just make sure the drain hole is clear and open.

It depends on how big the crocks are — 5 gal? 15 gal? Bigger? — and what you intend to plant.

Adding filler reduces weight and soil volume and can also raise the perched water level. If you want them moveable and are planting something that doesn't need all that soil, lightweight filler is fine. If you plan to install a shrub or small tree, you'll probably want more soil.

Either way, if your potting medium drains well and the crocks have drainage holes , water shouldn't collect in the bottom. But water in the soil can freeze and expand. Some clay and ceramics are highly susceptible to freeze damage — I think it has to do with firing temperature. No telling whether your crocks are high-fired or not. Are they vintage? Decorative? Designed to be planters?

Location makes a difference. It's actually better if they are not subject to constant freezing and thawing, so keeping them out of a spot that receives direct sun during the freezing months can help preserve them. If you're going to use them only seasonally, Catpaworchid's suggestion to cover them from rain is a good Idea.

In short, there are many more variables to consider besides the two choices you presented. I'm zone 7b and leave my large glazed ceramic pots out all year, some with soil and perennials, some empty. No problem.

I can appreciate wanting to fill some of the space so less soil is needed. But putting something that is virtually weightless @ the bottom will raise the center of gravity and make the pot more susceptible to blowing over in a gust of wind - if the plant used is tall enough.

If you're not worried about that, an empty plastic bottle with the lid on is much easier to deal with.

For the purpose of making the pot as heavy as possible, I used to put a big rock in yard pots when I lived where there were rocks. Here, I use a few broken bricks.

Consider looking for a plastic nursery pot that would fit neatly inside the crock.
This semi-cache pot method makes it easier to lift out plant and soil if you need to, and I think improves drainage- I repeatedly find my plants cannot drain well from heavy crockery (if there is only one drain hole)