I've used the mirror myself to get the base done. It's very helpful, but if you were to eventually do pieces, it wouldn't work out too well, since angles and perspective have a huge impact on pieces.
It's also helpful to give it a try without the mirror to get a feel for proportions and alignments manually, since those skill will eventually translate into a 3D space. Of course your are bound to get disproportions on the first tries, but keep in mind it's just exercices, don't stress too much about the end quality, just on what you are learning from it.