I've never looked at the problem sets, to be frank, but at the very least you'll learn just as much as you would doing them using Oracle. At best, you'll learn even more - where the examples include Oracle-isms you'll have to learn to translate them to Postgres, using the documentation on this site as a guide.
Along with the lessons learned by doing the problem sets you'll learn why those of us deeply involved with the OpenACS keep grumbling that keeping the port up to date would be a lot easier if the SQL were written using some Tcl to abstract out some of the differences (quite possible for all but outer joins and "connect by").
So I'd argue you'll learn MORE doing them in OpenACS. Whether or not the extra knowledge is worth having depends on what you plan to do later. If you go work for aD you'll work strictly with Oracle and expected to enjoy it, for instance.
Of course, instead of learning to install Oracle during the first problem set, you'll learn to install Postgres. This is a great simplification. You should probably go through the exercise of installing Oracle once just so you can have a proper appreciation for just how easy it is to install and configure Postgres, a strong argument for using OpenACS when a site doesn't really need the scalability of Oracle.
