Even if it's doesn't leave the right shine on the decals I rather have that then peeling decals.
On these two I've used Modelmaster metalizer sealer and it hardly yellows at all.
The Opel is painted with Humbrol gloss black, decaled and MM clearcoat.
The Mercedes is Modelmaster enamel, decals and MM clear coated.
...and finally the Volvo that also has a Humbrol enamel, decals and MM clear coats.
The Mercedes is about 5 years old by now, the Opel 7 years and the Volvo is 10 years old. No one of them suffers any bad yellowing, a slight hint on the Volvo that is only evident if you compare it to a non clear coated area.
So my suggestion is clear coats if you want to make sure the decals is still hanging on a couple of years from now. On the other hand I have builds outdating these three without clear coats that still has their decals intact.
Niclas