I am going to partially disagree with GNUtux. But first, I would state that downloading a collection of scripts is not the best idea (and I think Mr GNUtux would agree). Take the time to get each script from the original author of the script; from their own website or from a website to which they themselves have uploaded the script.
When you download a large collection of scripts from a third-party, you are relying upon that third-party to provide support for and to maintain those scripts. If you encounter problems then the first place you should go for help is to that third-party.
As to the issue of whether the scripts in such collections should be renamed, I think they
should be renamed; but also that the names of the globally defined function within the script should
also be renamed. The problem that FX_foundry had was that it renamed the files containing the scripts, but it did not rename the functions. This is what led to conflicts should the original script ever be installed alongside the one from the collection.
As near as I can tell, Mr Sherman has made the effort to avoid such conflicts -- if he modified a watermark script, he would not only change the name of the file but would also change the function definition within from "script-fu-watermark" to "FU-watermark". This should avoid the conflict which GNUtux mentioned.
I would note that Mr Sherman's efforts may not always be complete, nor successful. I did not look at but a small sampling of his scripts but I notice that my own
Calendar script would conflict with the one provided by Mr Sherman's collection, but this would be a somewhat exceptional case as I had more global definitions than just the two functions provided in the menus (a typically unsound thing to do on my part, but I had my reasons). I imagine Mr Sherman would be happy to change this should I request.
For myself, if someone does modify a script that I wrote then I should hope that they would change the name (I follow a naming scheme of placing my initials in the name which I think facilitates this). I have no problem with people changing my scripts and distributing them, but I don't wish to deal with the problems that might arise. I'd personally rather not have my scripts included in collections but as long as I know that they are not my own supported version*, at least I can avoid wasting my time debugging some subtle change that was made by someone else.
Obtaining your scripts directly from the author, or from
GIMPscripts, is preferable to getting them from a collection, but
if your going to download a collection of scripts then Mr Sherman has a good track record of supporting what he supplies.
* All of my own scripts which I actively maintain are available
here. I have submitted a couple of scripts to GIMPscripts and am committed to supporting those as well.