Is it necessary to sand before the last coat of oil paint?
It depends ;)

I don't tend to do a final sand for the very reason you mention. However there have been times when the finish hasn't been to my liking so I have sanded - but in these cases I use the finest grade paper and sand very lightly to just take off the drips and/or dust flecks that have settled on the wood.

So if the penultimate coat is good you might be OK without sanding, but sanding does provide something for the final coat to bind to if the previous coat was applied some time ago, so ultimately it will provide a more durable finish. If you are painting the second coat as soon as the first coat is dry then you don't need to bother.