When I had my custom article converter built, I used underscores inbetween title words to replace spaces.
Because every other Adsense website creator I’d seen used underscores?
Probably because the first adsense site creator started doing it and everyone else thought he must have had a reason.
But, as it turns out, hyphens beat underscores when it comes to keywords in Google.
Matt Cutts wrote in his blog about this exact issue:
With underscores, Googleâ€™s programmer roots are showing. Lots of computer programming languages have stuff like _MAXINT, which may be different than MAXINT. So if you have a url like word1_word2, Google will only return that page if the user searches for word1_word2 (which almost never happens). If you have a url like word1-word2, that page can be returned for the searches word1, word2, and even â€œword1 word2â€³.
So, I’m going to head back and change the title format from underscores to hyphens.