Where to install bootloader when installing Ubuntu as secondary OS?

Here's an example that could help you out:

Under "Device for boot loader installation":

• if you choose dev/sda, it will use Grub (Ubuntu's boot loader) to load all systems on this hard drive.
• if you choose dev/sda1, Ubuntu need to be manually added to drive's boot loader after installation. (for example, you previously have Windows installed on another partition of this drive, you'll need manually add Ubuntu into mbr)

Source

These two could also be relevant to your question:

• How to make Windows the default OS in grub menu?
• http://ycode.blogspot.com/2012/06/ubuntu-1204-boot-windows-by-default.html

Out of my experience:

• if the boot system is legacy/BIOS, choose the device /dev/sda straight away. Even suggesting other possibilities had been earmarked as a bug (https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1049549)
• if the boot system is UEFI, choose the partition where the UEFI files are stored, with a name /dev/sda. where the dot indicates the partition number applying to your situation. You can launch gparted and see which partition this is upon looking at the column 'Flags' and at where the tag 'boot' is.

These solutions had me reinstalling Ubuntu 14.04 without compromising the dual-boot menu at start up.

If you are uncertain about the sort of bootstrapping in your machine, consider the post Is my Windows installation booting in UEFI mode or legacy mode?

For general information: https://help.ubuntu.com/community/Grub2/Installing