Furigana is present in non-tank raws (ie magazine raws) too, actually, but typically only for shounen and shoujo manga. Very rarely (or never) will you see furigana for seinen and josei manga regardless of their source, except for extremely uncommon/difficult words. This is because shounen and shoujo manga are aimed at a younger audience who may not be able to read even some of the simpler kanji.

Furigana would be very helpful since it enables you to be able to check a dictionary quickly and easily to find out the meaning of that word, but you also need to have some basic understanding of grammar at least to be able to have an inkling (note: not a full understanding) of what the sentence is saying.

Reading a manga without having a decent command of the language is possible but you'll only have a partial understanding. It's good practice, I can vouch for that since that was what I did when I just started picking up Japanese, too. But don't even try translating because chances are 80% of it will be incorrect and you'd just be misleading those who don't know any better.