Well, it's not so much as necessary as it is more ideal.
If you prefer to do the texturing first, by all means, go ahead. I only suggest to do the trackwork first because it seems that scenery work is so time consuming that the actual route in terms of what's laid down is not much when a lot of effort has gone into it. On top of that, I find that a route built around the track means there's less to edit and that the landscape is more likely going to suit the original track plans.
If you do do a large route, doing basics would be fine. I just suggest to be careful in managing what gets done first and prioritize.
Cheers,
Joshua