It depends on the paint. For he best results it is important to apply decals over a gloss surface which is smooth. Matt (flat) finishes, which many model paints give, are in fact not smooth but rather rough at an almost microscopic level. This causes air to become trapped under the decals and this is what leads to 'silvering' of the decal film, causing it to become visible. To avoid this most apply a gloss varnish over the painted surface before applying decals. This has the added bonus of offering some protection to the paint too. After the decals are applied another gloss coat will seal and protect them and prepare the model for any weathering. A final coat of varnish may be required to give the desired finish to the completed model, be it satin, matt or whatever.If the model has been finished in gloss paints then that initial gloss varnish, to provide a suitable base for the decals, might not be required.
Cheers
Steve