I am with mario on this 1.
We have a path to follow and any changes in a time line could screw up the present as we see it. I am sure there is a word for it which i heard on star trek a while ago.
Say for example you 'created' gun powder to try and stop an event which has already happened (because you went back in time) a huge arms race we come (its a fact we are far more creative in times of war and most major break threws in technology is because of war).
Say you tried to increase hygiene at an earlier point in OUR time line, would we not all be in NBC type suits way before our time?
I dont believe in fate and dont believe that a path is laid out already for us what i do believe is that no 1 person or group of people have the right to try and decide the future for everyone else and that we must take each day as it comes