I disabled tooltips on the panel and it worked for every applet...except the pager applet, which continued to display the tooltip balloon. Doesn't look like this app honors the tooltip attribute.
Hrm, well the reason for this is that the panel setting only affects the single tooltip displayed by the panel on behalf of applets, and desk guide displays its own tooltips. There's a setting in desk guide properties to control the desk guide tooltips; I think that's the best we can do for now due to backward-compatibility constraints, a future GNOME version may offer a more satisfactory solution.