You should get the stump ground out deep enough that there is no wood in the ground under where the stump was. All of the chips and loose dirt should be removed and gravel should be put in the hole to bring it up to grade. A little rebar in the concrete patio over where the stump was would be a good idea also.
The three most important steps in pouring a cement slab are: preparation, preparation, and preparation.
If there isn't good, well drained, compacted substrate, under the cement, it will crack and settle or heave.
An old stump, or even stump debris, will rot and become a void under the slab. Without support, it will fail.
Grind your stump, and remove stump debris, then pour the slab.