The following is a value judgement on stepping stones: Whether you install them before or after sodding, lawns have a way of overgrowing stepping stones and they are hell to trim around, tilt and rock over time, get mossy, and are easy to trip on, albeit quaint and rustic looking. Have you considered putting in flagstone walkways, with close-fitted puzzle pieces style, proper curving edging, and compacted baserock and sand base? Put in that way, they last for years, are easy to trim around, drain properly, and do not tilt. There are lots of how-to books on pathways in the big box stores, and I heartily advise you learn well what is needed, so even if you farm it out, you will know how to specify and supervise the work.