You have to make qt.fontguess under /etc or .fontguess under home directory for using font guess feature in Qt. The file format is as below.
[jisx0201.1976-0][jisx0208.1983-0] helvetica "dfpop sb" "dfpop sb" courier "dfgothic md" "dfgothic md" times dfgothicp dfgothicp lucida 'fgheiseimincho w7' 'fgheiseimincho w7' agate dfgothic dfgothic
Please surround the font name that included space with " or '.
More info is this page.
Home of Japan KDE Users' Group (sorry Japanese only)