Question on multi booting and grub

I have a linux aao and want to multi boot keeping linpus and adding xp and another linux. I have multi booted winxp and ubuntu on both my desktop and laptop for some years always with the same method. ie I install xp first and then ubuntu choosing to install grub to the mbr. Obviously now I want to reverse this and install xp when a linux (linpus) is already on the drive.

I have read the 'Idiots guide' thread on here viewtopic.php?f=47&t=8944 but have some questions as it seems to handle grub differently. From my reading of it it suggests changing the boot flag to the win partition to install xp. Then to get back to linpus just changing the boot flag back and editing grub.conf to add xp and then selecting os via grub. My experience of installing grub to the mbr was that if windows is then installed it overwrites the mbr and to get back to linux meant reinstalling grub. Does this mean that the Linpus grub is installed to the linpus partition and not the mbr and this is why simply changing the boot flag gets back to linpus? This does seem easier that reinstalling grub.

Next, when I come to install the second linux os (triple boot) what is the best way of handling the grub install for that os? I assume I could install it to the mbr and let it handle the linpus and xp systems but I like the idea of updating the linpus grub and using that in a similar way to adding xp in the guide. Should I hence install grub to the second linux partition or not install it at all? (I can't even remember whether not installing it is an option).

thanks