Despite the fact that a 27 speed bike is advertised as having 27 gears, you should really only use 11 or so of those gear combinations (lowest front gear with rear gears 1-3, middle front ring with rear gears 3-7, top front ring with rear gears 7-9). The chain should run basically straight, if it has to bend a lot (such as using the left most rear gear with the right most front gear) you'll wear out your chain faster as well as increase the chance of the chain slipping onto another gear or even falling off entirely.