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.