i think it's easier to learn it in the waking life. if you don't really know it in the real world you won't be able to know it in the dreaming life either.
let's say you haven't seen mona lisa, and then you decide to have a LD about how she looks. you won't get the real image then. but if you know how she looks then you wil be able to get the picture.
if you learn the periodic system so you know it but still a little bit unsure you can go lucid and practise.