If you're really that fussed about wasting space on a 23 pence* DVD-R then use a DVD RW so you can erase and start again. Or write to the DVD-R in multisession mode instead, then you can write more.
Better to do either of the above than to overburn, which may make CDs/DVDs unreadable on some systems. If your current writer goes bang, where is the guarantee that you will be able to read the disk in another drive?
*
http://www.tekheads.co.uk/s/product?product=608647