This has turned into a fascinating thread. I've been wondering myself if I should level gathering classes (Botany and Miner first) to 50 before tackling crafting. I wouldn't have to break up what I'm doing for the sake of gathering mats. In the beta I noticed a lot of people /shouting for mats like elm/yew logs which are around level ~16-20 Botany. The HQ versions sold pretty well for me on the Market Wards. For those people who can't stand gathering, there is a profit to be made with selling leftover materials. The lesson to take away from here is to make a profit by putting in work that others can't be bothered to do. Selling crystals/shards/materia/catalysts/spiritbond gear/high level mats will be profitable enough.
It would depend on the tradeskill you've set your goal on.
For Botany, if you plan on leveling Carpenter, I would heavily recommend leveling up Carpenter with it concurrently instead. This is because if you can provide your own logs, you have an extremely easy leveling method to use for Carpenter, so your sessions end up being something like "Get Botany session done, use proceeds to level up Carpenter 5 levels."
Everything else is definitely a YMMV kind of situation, like the bottomless pit of Iron ore that Armorer and Blacksmith need.