I agree with Donnie regarding targeting local or long tail phrases when looking for keywords to optimize for.

For example, if your client is a plumber, you'll have a hard time optimizing his site for the keyword plumber which has a difficulty of 63% according to the SEOmoz Keyword Difficulty tool.

Since your client is a plumber, though, you could include a local word like the city or town where your client is located and get better results. So seattle plumber has a difficulty score of 41%.

You can often add qualifiers to get commonly searched for long tail phrases for even better results. So best seattle plumber service has a difficulty score of 37% which you could probably get ranked for if you include it in your SEOmoz campaign and focus on it.

While it's not a complete waste of time to go for words that have higher scores than 20s or 30s, you'll probably get better results faster if you go for words with lower scores.

I hope that helps!