Yes, you should be able to opt not to install Grub; however, do remember to run "sudo update-grub" in the Linux distro from which you installed Grub into the MBR, otherwise you won't see LMDE in your Grub menu during bootup.
JasonLG wrote:GRUB(the first stage any way) is installed in the Master Boot Record which located at the very beginning of your hard drive. /dev/sda is the correct place to install it if you want to boot an OS that resides any where on your sda hard drive. You are not presented with any other choices because I assume you only have one physical hard drive.
No, Grub does not have to be installed in the MBR. You could be using LILO, GAG, or even Windows' own boot manager...for the latter two, you would then have to chainload to Grub in order to boot Linux. Point is, something has to be installed into the MBR, but it certainly doesn't have to be Grub. Granted, most people are going to be using Grub and have it installed in the MBR, but it's up to the individual user to decide.