I've installed and used literally dozens and dozens of rod holders on various boats and have just screwed them in - all still in service. But note that none of these are/were ever meant for heavy duty use. This was always into relatively thick fiberglass, as well. However, it certainly can't hurt to take extra precaution and use the backers.
By "should" I "should've"
been more precise and said there "won't" be anything there. I've worked on enough boats (I've worked in this business full time/part time for 30 years) to know there is no logical reason to mount anything underneath that area. The only thing even in that area is that (I assume) black wash down sprayer... or whatever that is. But it's electrical wires will not be run in that area.
Besides, the fastest/best way to find out, for sure (which is always still a good idea), is to just give a quick look as it appears there is decent access.