your tripod should be rated to carry a certain maximum weight, which is perhaps the fist thing worth checking. Just as a starting point anyway, because it will tell you if it's reasonable to put that combination on the tripod to begin with.
However, it's unlikely that any tripod will not support the combo you have. The Xsi is pretty lightweight in and of itself.
As a rule of thumb - if it feel unsafe to lift the camera and lens by the body - i.e. if it feel too front-heavy to seem safe and you have a concern that there would be too much pressure on the lens-mount of the body, then you should mount it by the lens.
I've not personally checked the manual for my Nikon D40, but I suspect that it does have some guidelines for how heavy a lens one should mount and support by the camera alone. I go by feel.
The XS - as well as the D40 - are entry-level cameras and as such, they are probably the least sturdy bodies you can get. You may want to err on the safe side.