Here's what I think is going on. (I'm ''not'' a native speaker, so fair warning.) First, this is the first time Ive come across the idea that you use soll for instructions and sollte for suggestions. I suspect it's less a rule and more of a guideline. In any case, German grammar rules are notorious for having many exceptions, caveats and subtleties.
I do know that the subjunctive is often used to soften requests and instructions. So I can see how one might, in general, tend to use the subjunctive for advice and indicative for instructions. But a lot would depend on the situation, the particular phrase being used, and other factors. In this case, the adverb lieber is used in the first sentence, which already softens it. So it seems like it would be going too far to soften it more by using the subjunctive (see vonbrand's comment).
English has a number of ways to soften instructions as well. For example you could say "You might want to take showers to save water," or "Please consider taking showers to save water," but combining the two, "Please consider wanting to take showers to save water" sounds odd. German has different ways of doing things like this, but I think the idea is similar: not softening instructions at all can sound impolite, but you can also soften them too much.