the .Xresources file is in the format classname*paramater: value. Once you change it, you need to either log out or run xrdb -merge ~/.Xresources.
You should be able to set .Xdefaults to do much the same thing without reloading.
To find the class name, use the xprop utility to look for WM_CLASS, e.g. for xtightvncviewer
ianw@mingus:~$ xprop WM_CLASS [click on xtightvncviewer window] WM_CLASS(STRING) = "xtightvncviewer", "Vncviewer" ianw@mingus:~$ cat ~/.Xresources xtightvncviewer*grabKeyboard: True
Most applications use a client name that is the executable name with a capital letter, but xtightvncviewer doesn't quite follow this rule.