I would define my style of RTS play as "teching" not "turtling". "Turtle" I have always understood to be a defensive style of play, relying on outward defenses and not engaging the enemy. "Teching" however, I believe to be about intentionally not engaging the enemy for a certain amount of time, until research has been done, resources have been collected and an army of superior units can be produced. It's playing smarter, not harder, and is often less wasteful of resourcs.
For instance, I play Supreme Commander; Forged Alliance with my boyfriend. He's a rusher, I'm a techer. He regularly runs out of resources and needs me to back him up. I will support him, and then when I am ready, will regularly single-handedly destroy all remaining enemy units. It's not to say either tactic is better; in fact we work best together and in a multiplayer situation are often nearly unstoppable with our rush/tech approach.
Some RTS games are more suited to teching than others. Warzone, Forged Alliance (actually, ALL the Supreme Commander games), and Age of Empires are among the most tech-friendly. Among the less would probably be C&C games and Starcraft.