I do. I have a terrible time knowing when to start teching up.
I typically start with 4 gate and get some units up. once 8-12 units are up I begin pushing out. I poke in, see if those initial units are still their, if not I know hes teching up and wont have many units. Thats a clear sign for me to do one of two things. Expand or drop 1 of 4 tech buildings, a Forge, Robotics Facility, Stargate, or Twilight Council. Lets assume I am up against a Terran player. I tend to Expand because it is relatively safe if my army is outside his base and preventing him from moving out. With only half a dozen marines at his cliffs, that tells me hes either going for some tanks, or Banshees. More than likely its Banshees.
Do I drop a Robotics Facility down instead of expanding?
No, I still will expand, because getting that 2nd base up and going while preventing him from getting his is so important. I'm now in a position to know what hes probably doing, and I have now limited his choices. He can either continue with Banshees and get cloak and probably hold his ramp off if I decide to push in again, or make some more ground forces to push me back. Both are perfectly reasonable. Most players I have faced tend to continue with their Banshees and get cloak. Its important to poke up his ramp to see whats their, as if some Marauders suddenly pop up, I know hes looking to buy time before cloak is ready and hes going to make a push a bit later with not only a force, but also Banshees.. That means I need to have dropped a Robotics Facility sometime after my Nexus gets placed.
See how important Scouting is? I began scouting more frequently and accurately because I primarily play in a reactionary way to my opponent. Because I do this, I need to be knowing what my opponent is doing pretty often. If I see some heavy Marauder play I need to grab an Immortal or a lot of Stalkers ASAP. An observer would also help as well as these provide a way of keeping track of your opponent without having to sacrifice the occasional probe up his ramp.
So what things do you wait for either in your build order, unit count, or what your opponent does to justify teching up?