Question about shellac

I'm refinishing some old cherry and mahogany windows. The rails and stiles have been scraped clean of the old paint and multiple layers of varnish. The surface is lightly sanded and the wood stained so all surfaces are similar in appearance. I was always taught to apply several coats of shellac to seal the surface and then to apply the spar varnish. Recently I was told the shellac was completely unnecessary and a waste of time. What does the thundering herd think? Shellac first or direct to varnish?

Shellac is a sealing top coat. It is not always necessary, it all depends on the particular circumstances.
In your case, old wood with unknown products in various layers.
If you like the uniformity of the color and finish you have, I would seal it with shellac first. The shellac will create a barrier that the new varnish will adhere to and should not affect the underlying old varnish. This will help to insure nothing bleeds from the old to the new and vise-versa.

I agree with Edward, especially since you indicated you stained the wood. A thin barrier coat of shellac often has benefits in that respect for sealing the stain and providing uniform adhesion to the top coat. Note I used the words "thin barrier coat". Shellac should always be used in the thinnest coating required to do the job...it should not be built up like varnishes and other coalescing finishes as it "dry"s via evaporation, not curing.