Yes, it makes a difference, depending on how you do it. There are two
versions of grub, legacy (old) and grub2. Grub legacy lends itself well
to using a text editor. Grub2 can also be editted with a text editor,
but it is more complicated because you are not encouraged to do it this
way, and it is not permanent. An update to the kernel for exam ple will
re-write the grub.conf file and it will not include your manual
editting. So if you are just text editting then it does not make much
difference.

However, if you using grub2 and are doing it properly then it does make
a difference as it will generate a new grub.conf and it will be very
different from just a text edit. It will be re-built and be completely
different depending on the distribution. For starters the default
distribution will change and depending on the distribution it will do
things differently. For example Fedora seems to ignore anything but
Fedora. Mandriva and a few others only include that distribution and
then allow you to manually add separte grub entries. Finally some
distributions will use grub legacy while others will use grub2 and even
lilo. I have found so distributions are very accurate and others are
very poor, and even mess up drive order and names.

Ubuntu is about complete and well rounded compared to many
distributions. It detects all operating systems and makes an entry that
actually works. I would use it and depending on your setup, I would
favour generating a new grub.conf from scratch over just text editting,
unless you ahve good reasons for not doing it this way, for example
removing references to kernels that you don't plan on using. It would be
better to actually remove the kernel though using grub and then it will
change grub automatically.

I wonder why you would choose to have two instances of Maverick, one for
Ubuntu and one for Kubuntu instead of running them from the same grub
entry and changing it a login which is the way that I do it. I just boot
Ubuntu and change to KDE, LXDE, XFCE or GNOME from the session menu.
They all run from the same partition. I would only have separate grub
entries if I was using a different version, let's say one was Maverick
and the other Lucid. Just curious. :)

(I know that you are not using other distributions, but some people are,
so I included my thoughts on those.)