If you are using these web links for short term convenience, go for it. HOWEVER if you are using them for long term you are creating a big problem. Web links are relatively short lived in the overall scheme of things. Most of the links I originally added to my data base have become obsolete after a few years. I have now removed them, and I do not use web links for references. (The database is over 10 years old)
For reference the most important thing is the Nemorial Number as that is probably basic to the database, and is highly unlikely that it will ever be changed.
When I find something I like on Find A Grave I either copy the information to FTM notes, or print the page to a PDF document. While I don't, you could attach the PDF document with the Find A Grave information to the person as media.
Remember the only thing carved in stone on a Find A Grave site is the information in the picture of the tombstone. All else is highly questionable like the information in all Online trees. As with any thing you must do research to find supporting documents for the information.