It all depends on what your current partitioning scheme is and what you want it to look like with Ubuntu installed. If you are familiar with GParted, I would highly recommend running that from the LiveCD and setting up partitions before starting the installer.

P.S. Oh, and I am also planning on not modifying the MBR of Windows if I ever do dual-boot with Ubuntu, using instead a piece of free software (like easyBCD or something) to avoid the headaches of Grub being overwritten by a Windows update.

In that case, use Windows' Disk Management to move/shrink all Windows partitions to the LEFT of the hard disk. You can then start directly with the Ubuntu Installer's partitioner -- create an extended partition in the free space, within which you can create all your Ubuntu partitions, and be sure to install Grub to the extended partition (typically something like /dev/sda5) and NOT to the MBR (/dev/sda).

am I correct to think that it would be the acme of foolishness to try to partition drives within a virtual machine

No. The hard disk in a virtual machine is like any other, so feel free to partition it, dual-boot your virtual machine, triple-boot it even! The only limitation is that of performance compared to a real hard disk, imposed by a combination of Virtualbox and the underlying physical hardware.