It isn't dangerous just changing timings, if by 'dangerous' you mean fatal to the machine. But if the settings are not stable, you may not be able to run without crashes or not even start at worse. To get the RAM to be stable, increasing the voltage may help. But there you get into an area where it can be fatal to the RAM or the motherboard if done carelessly.
I might suggest that you increase the freq of the CPU first. That will give you way more performance gain than the RAM. Another suggestion is to use matched pairs of RAM when you OC. Stability is really hard to get with mixed RAM. Also, using the single stick will in one of the slots will make the whole RAM system run in single channel mode instead of dual channel for about a 10% hit. Dual channel only works when both matching slots are populated. There are some exceptions to that, however. You really need to list the system specs.