I think you should probably retopo.
I feel that it's much more difficult to unwrap a high poly mesh because edit mode for very complex models may be less responsive, and it will be more difficult to get a good selection of edges for adding seams since the geometry is so small and there are so many edges
you probably want to retopo for your own convenience, and if you need the extra detail, you can bake the high poly as a normal map for the low poly model
if you really need to sketch out a simple texture on a very high poly mesh, maybe you can use vertex paint? it won't be as detailed or look as nice as a real texture, but if you need to visualize something for planning it might be useful