I don't believe there is a policy specifically for this although it has been discussed quite a bit here in G2G. I think everyone prefers any source to no source, and most people prefer a free source to a paid source assuming they are equally legible.
As far as linking to images of original documents or publications I can't see why anyone would object to links that point to freely accesible sites such as FamilySearch, Archive.org, Hathi Trust, as well as the many genealogical socieites, archives, museums, and libraries that are digitizing content. Aside from being free, we can reasonably expect that the information is not going to disappear.
FindAGrave on the other hand, like any 'user contributed' information (including Wikitree and personal websites), is likely to change or disappear entirely as editors of profiles see fit - there is no expectation that what is there today will be there tomorrow. I personally wouldn't replace a link to ancestry with a link to findagrave simply for this reason. Imagine you came back to your profile and found your working link to a document on Ancestry now pointed to the same info on findagrave but the link was dead because the content was changed. Again, not something I would do.
Another option is simply to put the links together [FamilySearch Copy] [Ancestry Copy] - if links break in the future now there is some resiliency.