What do you mean by "individual keywords" ? As I know there is no difference between strong and b tags, but strong is recommended by w3.org as the standard for bolding the text. Or better, in XHTML, where is CSS era dominating, font-weight:bold is in my opinion the most appropriate for using.
Actually, as far as I know, all SEs like bold text. Bolding keywords can help in getting better ranked for those keywords, but all that is just relevant and depends on many other factors...
Just my 2 cents
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