i have no 100% exact answer for you but im pretty sure it matters. why wouldnt it, and where is the line between kw's that are too important for it to matter. i have never heard of it before.
Alternatively you can go for "keyword phrases" which still matter even if it is highly competitive field.
If I understand your question corrrectly..having your keyword in your domain name is good SEO! If the niche is really competitive..I try to use longtail keywords or geo/target specific keywords (depending on your target market) example Money Making Tips For Stay-At-Home-Moms!