grub will figure things out for you so no real need to do anything with it if you want each OS to be included in grub. If you want multiple Ubuntu's on the same machine one method could be to just partition your hard disc for the amount of version your are going to install. Ubuntu 8.04 to 11.04 makes 7 operating systems so divide your harddisc up into 7 partitions and a data partition (you can use that to use the same files (audio,video etc). With the 1st one you format this datapartition. With the other 6 you just mount the datapartition.

It is time consuming to set this all up and during install you basically can not use your system. And it is not flexible.

There is another option that I would prefer:

Depending on what you want to achieve with this you can also use virtualbox-ose-qt for every operating system. This would require you to create 1 Ubuntu on the whole disc, install virtualbox and inside virtual box you create space to install the other 6 operating systems. If you do not want to test for speed this could be a more flexible option: adding more or deleting existing systems is possible without needing to reformat your disc or create more space to install it.
And when you are done you can simply remove all the operating systems and still have your base system up and running without the need to add the other partitions to this system.

Here is an example from kUbuntu with Ubuntu:

It is slower than normal and you need a bit of RAM in your machine but you can also run all the OS's at the same time if need be ;)