After setting up your website and you decide to change your meta keywords and or description to what you think would be best for your site, is that bad for SE's like Google?
donno bad or good depends on what changes u have done . i do change meta if i dont see any improvement in rankings in 3 months . if you are already ranking high then u may see drop in rankings it can improve too but wil tak time according to my experience
Well if you have a valid on why you desperately need to change in order to help on the traffic, then just do it anyway.
There's no problem changing them. Just don't change them often. Do your research now as to the keywords you should be targeting and get those keywords in your page titles and descriptions. Once that's done focus on link building with the same keywords.
If you change on your site from keyword x to keyword y you will loose ranking for x and improve for y. All with some delay.