I will first start with my path and then offer a little explanation afterward. My only rule is that I would thieve approximately five levels past the next requirement. I felt that it cut down significantly on the failure rate.
Level 1-15: Man (good for some initial cash)
Level 15-30: Farmer (great for beginning seeds to those interested in farming)
Level 30-43: Warrior (some more cash)
Level 43-60: A few options open up here. For additional seeds, you can start thieving the master farmer along with the seed stall in Draynor. It's close to a bank for easy deposit and a great source for seeds. If farming isn't your thing, then concentrate on guards. They are located in most major cities which means you are also close to banks for food replenishing.
***At level 60, I pretty much was confined to Ardougne as it holds the remainder of valuable thieving options***
Level 60-75: Ardougne Knights - I skipped watchmen here because with level 42 thieving you can steal from the food stall and at 50 thieving you can steal from the silver stall. I would mix both of those in (food for health and silver for crafting) to break the monotony of the knights.
Level 75-85: Paladin - great that these are in Ardougne, because the gem stall opens up at level 75 and is a great source of gems for crafting considering you thieve uncut gems. The Paladins have a high failure rate even starting five levels beyond the requirement.
Level (80)85-99: Hero - the only time I broke the five level rule was for Heroes. You can start these right at level 80, or wait until 85. These at first have an alarmingly annoying failure rate, but their loot is worth it. These guys drop: Cut Diamonds, Gold Ore, 200-300 GP, Wine (great to use for HP replenishing), Death Runes (2), and Blood Runes (1), and fire orbs. Early on with Heroes, you can expect rates around 100-125k per hour. Once your thieving gets into the 90's, expect rates to stabalize around 200-225k per hour with luckier sessions yielding up to 250k in an hour. Be sure to get that crafting level up because at level 70 crafting you can turn those cut diamonds into diamond ammies for 200xp each.
It's short and sweet, but it's the path I took for 99 thieving. If you have any questions, feel free to leave them here and I'll do my best to provide insight.