Yes, efficiency is a huge factor and very important for sizing gearboxes and designing mechanisms.
I use the following for designing using torque*:[ul]
[li]Straight Spur Gear with good bearing condition 95%[/li][li]Straight Spur Gear with funky bearing conditions 90% (look at the final stage of the FP transmission for an example of a funky bearing condition)[/li][li]Planetary[LIST][/li][li] Low ratio (<5:1) 85-90% (depends on how good the bearings are, the grade of the gears, the size of the planets w.r.t. their axles… things like that)[/li][li] High ratio (>7:1) 50%[/li][li] Very High Ratio (>20:1 stages) 15% <<these are great for speed reduction by division, but lousy for torque increases by multiplication[/ul] [/li][li]Helical Gear, parallel axis 80%[/li][li]Helical Gear, cross axis see Worm Gear[/li][li]Worm Gear – totally depends on lead angle[ul][/li][li]Best case 50-60% (high lead angles of 40 deg, good bearings, etc.),[/li][li]Worst case 5-15% (lead angles of 10 deg, bad thrust management, etc.)[/ul] [/li][li]Conical Gears / Bevel Gears depends on bearing arrangement and alignment 60-90%[/li][li]Chain 90% (assuming good alignment and tension)[/LIST]This is PER STAGE. [/li]
Example: If you have a 4 stage 4:1 per stage spur gear gearbox with good bearings it would be .95^4 = 81% efficient. So… …instead of getting a ratio of 256:1 your “effective ratio” (from a torque point of view) would be 207:1.
Continuing with the example, if you put a FP in with a stall torque of .45N-m then you would get 93N-m out of this gearbox, not 115N-m. Now suppose you are trying to lift your robot with this gearbox and you have the output connected to a .17m arm (and assume your robot weight is 600N, then you need 100N-m to lift your robot.
NOTE: You are not going to lift that robot, all you are going to do is turn a lot of electrons into heat.
Continuing, if you put a 3:1 chain stage between the arm and the gearbox, the effective ratio would be 560:1 (207X3*.9). You could put 250N-m of torque on your arm. Now your motor would be loaded at 40% of its stall during your lift (and the motor would be running at 60% of its free speed or the arm would be turning under load conditions at 12RPM = 16,000RPM .6/(2563)<<Note: Actual Ratio used for SPEED, Effective Ratio used for Torque).
Now you’d lift in a heartbeat (1/2 turn in 6 seconds – well… …kind of a LONG heartbeat 
and you have extra torque should another robot get in your way on the way up.
Life is good… …always.
Joe J.
*Some say I am too conservative but my experience with FIRST and with automotive actuators tells me that these numbers are not far from the right ones.