The answer to that depends on your preference and possibly on the specific items in question.  If an item is both shipped and sold by and also by 3rd party merchants, are you okay with them being fulfilled by a 3rd party merchant if fulfillment is not available or the 3rd party merchant price + shipping is cheaper? If you are open to using 3rd party merchants, then you would set the items to All Sellers.  If you are not okay with using 3rd party merchants, then you would set the item to Walmart Only (Default).  The regular "Default" should never be used because it's going to toggle back and forth between the two.  All items from Walmart should either be set to Walmart Only or All Sellers depending on your preference.

If you're using Sku Fetch, then you can set a global setting of either All Sellers or Walmart Only in your Filters and Formatting Settings.  So Sku Fetch will automatically apply that to your listings when they are exported to Sku Grid for tracking.

For all existing items that you have in your Sku Grid account, you can update them to either All Sellers or Walmart Only in bulk using the following process -

1. Essentially, set your grid to view 1000 items per page.
2. Select Walmart from the 2nd set of Filters on your page so that only Walmart items are displayed.
3. Use Multi Item Actions --> Select All on Current Page
4. Then Multi Item Actions --> Update Store Options --> Walmart --> All Sellers or Walmart Only (depending on your preference)
5. Then Multi Item Actions --> Recheck Selected ASAP
6. and Finally Multi Item Actions --> Force Update the selected items
7. If you have more than 1000 Walmart items, you would then click on Page 2 and repeat the steps 3-6 above.

Going forward, when adding items into Sku Grid be sure to select either Walmart Only Or All Sellers from the drop down that says Default under the Walmart Supplier URL.