If it is the ZooMed heat table I have talked to a lot of people and a zig-zag pattern works the best. I am going to buy the heat tape from Bean Farm. The ZooMed heat tape has at leaste 6 feet of cold lead that will not heat and with 2 kittens in the house I will not take a risk of them pulling the cable.
I have heat cable for my one rack and I like it. I simply taped it to the bottom of each shelf with foil tape. I didn't do a zig zag pattern, instead I simply ran it twice per shelf. Be careful not to let the cables touch each other while hot or else they'll short (it happend to me!)
I am using the Zoo Med heat cable. Got if for 1/2 price at a pet store. I have the 11.5 ft cable (6.5 ft is cold). I tried just running it twice on one shelf and taping it with foil but it didn't get hot enough. So I ran it in a zig-zag pattern (5" lengths 1.5" apart) - it is enough for the one tub I have in it so far. (see it at http://www.geckoforums.net/showthread.php?t=16422). I use a dimmer to control the heat, since it is only one tub.