All applications - GTK+ and KDE(Qt4) look the same - fonts smaller than 12pt are not antialiased and fonts bigger tahn 12pt are antialiased, only for U++ applications all font sizes are antialiased (with my current settings). And this is under KDE, LXDE and Openbox.
When using Gnome or XFCE4 U++ applications look like the oter applications it seems that there is something running (like gnome-settings-daemon) which fixses it.
and now U++ applications look like the other applications under all desctop enviroments I've tryed.
It looks that when gtk_antialias = -1; gtk uses the default settings of X server.
...which is sort of problem, because you can change the setting in Gnome to affect that.
Thus I believe that probably the right solution is indeed to detect whether Gnome desktop is active or not and put there -1 only if it is not...