Is the reputation/prestige of the scientific journals where you published a paper really important?
I can say for sure that the prestige of a journal can help you, particularly if you are a young academic!

I was applying for a position at a university several years ago and ended up being the second-choice candidate. Even though my publication record was larger than the other candidate's, and I had been publishing in good journals such as Journal of Physical Chemistry, Journal of Chemical Physics and Physical Review—all of them highly reputable journals in their individual fields, I lost out to someone who had published several papers in Nature. (I learned this after the fact from one of the members of the search committee.)

So, yes, publishing in good journals is very important when you're starting out.