You don't exactly need a grid actually. If you don't need to get information back from tile, you can create a fake tile illusion by "snapping" objects to certain floating points.

For instance, say, you want to have a grid that has placement points on 0, 1, 2, 3 and 4.
Then when the object is at 0.3f, you round it to 0;
and when it is at 0.7, you round it to 1 (etc.).

This way you will create a snapping effect that gives illusion of a grid.

You don't exactly need a grid actually. If you don't need to get information back from tile, you can create a fake tile illusion by "snapping" objects to certain floating points.

For instance, say, you want to have a grid that has placement points on 0, 1, 2, 3 and 4.
Then when the object is at 0.3f, you round it to 0;
and when it is at 0.7f, you round it to 1 (etc.).

This way you will create a snapping effect that gives illusion of a grid.