Everyone does it differently. And I think it really depends on what your modeling. If your modeling something like a battlemech, that does not have any schematics front/side/back, you can just use basic geometry to start getting the shape you desire. If your modeling a rifle, or something you have pictures of front/side/back, you can put those pictures in your viewport and model them poly-by-poly.
It is very cool watching someone do a model poly-by-poly in a speed/model video, but as I said it isn't the case when doing battlemechs, usually. What I do is start with basic geometry and go from there. I'll start with the torso, cockpit, arm and when I can use symmetry on stuff that is left/right ie: torso, cockpit. That way I don't have to do everything twice, for instance the arm of a battlemech I can just do one side and just use symmetry for the other.
There is a catch to this though, you must eventually detach the arms and than detach the pieces and parts. If you have specific pivot points it will take those from the original arm, so you must have a way to get that pivot point to the newer part. One way to do this is always begin your model at 0,0,0 coordinates. This way if you use symmetry you know how far off you need to offset something when you detach it and need the pivot point to be fixed.
For me though, proportion/scale is one of the biggest hurdles when doing battlemech modeling. Because we have no set proportions/schematics allot of the time it comes to just eye-balling things. I'll sit for a few hours modeling something and than realize it is to small/big, but that is what the scale function is for!!
Bottom line, is it is up to you the artist to find the method that works best. I do use 3dsmax, but you can model in whatever program you have. Each program just allows you to do the same things in a different route, and some are easier than others to do those things in. Hope this helps, and you can always find me on the 'Outreach' community teamspeak 3 server if you just want to talk modeling!
