Re: back-driveable vs. not
 

If you simply need to keep a platform raised, or something to the effect of that, you might consider keeping a motor running just enough so the torque holds it in place, but does not change the position of said platform, etc.

Re: back-driveable vs. not
 

Just be mindful and use the correct diameter cable (if you are using cable). There is a definite working strength with cable, and it depends quite a lot on how you fasten it and the radius of the attachment used for the cable or whatever diameter pulleys are used. Leave a reasonable safety margin.

A 3/16 inch cable could be used with plenty of safety margin, or you could snap a cable of 1/8 inch or much larger if you don't engineer it correctly. If you have a cable running around a 1/4 inch bolt or something small, do you realize what stress that causes on the cable? Anything less than a couple of inches diameter should be carefully considered, even if you are oly lifting the robot a few times in the course of competition. Do be careful!