I would suggest doing so for several reasons:

1. decals adhere better to a glossy surface, so if you are using decals, you need to put a gloss coat down beforehand.

2. It will protect the paint from other steps. Weathering solutions, handling, and even decal solutions can slightly alter the look of the paint, so haveing a clear coat on helps protect the paint.

3. Washes flow better across a satin/gloss surface. You can still make them work on flat surfaces, but satin helps.

4. Finally, most varnishes have some UV protection, so it will help if you plan on displaying the model. It will also make it more durable in case you need to brush off any dust.