There's a considerable amount of debate about that among photographers.
Drymounting looks good, and it's what many of us are used to, but most archivists and conservators will say that drymounting is not archival, because it is not easily reversible, even if you use "archival" drymount tissue that can be released by putting it back in a press. Imagine that 100 years from now you are a conservator looking at a drymounted print of historic and monetary value, and mildew has made its way into the mount board--do you want to heat it up in a press to see if the drymount tissue releases?
Many will argue that the tissue forms a barrier to pollutants in the air and that the backing board physically protects the edges of the print, and these facts may be true, but by definition, "not reversible" is "not archival" by current standards.
That said, a drymount press is still handy for flattening prints for hinge mounting or mounting with corners.