So, a few of my own thoughts on the matter: First off, the main advantage from leveling up a craft is, funnily enough, convenience. With DoL/DoH leveled up to 50 you won't be reliant on the market boards to get items you want, and this can help you when the item just plain isn't listed (Like when filling in gaps where dungeons didn't drop loot, or assembling a full set of matching armor) or if the item is drastically overpriced (Like, say, any item soon after it comes out).

As for advice leveling, you may want to check out the GamerEscape wiki at http://ffxiv.gamerescape.com/wiki/Main_Page . You can use it to see what recipes a given item is used for, as well as how to obtain it. Using this, you can easily figure out if you should hold onto an item (You may use it soon, or it's difficult to re-obtain) or sell it.

You may also want to check out the list of all the levequests at http://ffxiv.gamerescape.com/wiki/Category:Tradecraft_Levequest . The link listed before seems to have a preference for leves where you can turn in the required items up to 3 times. While that DOES give the most xp per leve, it requires you to craft 3 items instead of one per turn in (or 5 instead of 3 in the case of recipes that make 3 per synth), and each turn in gives LESS xp than a standard leve which only requires you to craft a single item. So, it's a trade-off between what you value more: Your leve allowances, or the cost of materials and the time spent gathering them and/or crafting the many items.