You don't have to use a clear coat, and on a tank you might not want to do so. If you use a "sludge wash" technique, the clear coat is recommended. It depends on the wash technique and your intended results. Your wash will settle into the rough surface of the flat paint, changing its color subtlely. (Or not so subtlely if you use a heavy wash.) You may want that effect, or not. You can also clear coat some portions (simulating fresh or unweathered paint) but not others.
The point is that once the oil wash is applied to a flat paint surface, it will be practically impossible to remove all of it if you don't like what you've done.
The order you use different weathering techniques depends on the effects you want to achieve. Where I want a surface to look worn and dirty, I apply washes directly to the flat paint. Where I want the wash to collect around detail only, I usually apply the wash over a clear gloss.
I prefer to seal my weathering efforts with a clear coat. The level of gloss depends on the model. For a tank, I'd use a clear flat.