If you use a real partition for VirtualBox you shouldn't be able to write on the MBR of the disk, so it isn't a real danger in doing that unless you specify the entire disk or the wrong partition.
According to http://www.virtualbox.org/manual/ch09.html#rawdisk, the guest's MBR is written to a file instead of being written to a partition.
In some configurations it may be necessary to change the MBR code of the created image, e.g. to replace the Linux boot loader that is used on the host by another boot loader. This allows e.g. the guest to boot directly to Windows, while the host boots Linux from the "same" disk. For this purpose the -mbr parameter is provided. It specifies a file name from which to take the MBR code. The partition table is not modified at all, so a MBR file from a system with totally different partitioning can be used. An example of this is
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -mbr winxp.mbr
The modified MBR will be stored inside the image, not on the host disk.
The created image can be attached to a storage controller in a VM configuration as usual."