For example, would you mind it if your artist sold prints of that work? (And other such scenarios)
Don't do it
ever unless it's specifically stated in the contract that you keep the right to do it, same goes for any kind of merchandising.
What we usually do working with private clients (including self published artists) is to offer them non-exclusive rights to selling prints and merchandise bearing the image, meaning that both the artist and the writer can do it. If a client wants exclusive rights to prints and such they pay more.
In work for hire, any rights that aren't specifically stated as remaining with the artist are understood as being given to the client. In a worst case scenario the client isn't even obliged to credit the artist (although almost everybody does), the author of the illustration is interpreted as the means through which the idea of the owner of the intellectual property is put on paper.
In short, don't take for granted what isn't written down.