If oil is leaking on the driveway, If it were mine I'd fix it.
And no, if your valve covers are leaking, they can cause a fire at any time. If your valve cover gaskets are original, I'd start with replacing them. Its an easy DIY job if you have a few hours and gaskets only run about 50 bucks for fel-pro replacements that includes new bolt grommits.
Here is what can happen if your valve covers are leaking and your don't replace them... (this is best case scenario where the owner caught the fire before it engulfed the entire car).
http://www.automotiveforums.com/vbul...d.php?t=974654
An apartment building near where I work was actually burned to the ground because a GP owner who hadn't replaced their valve cover gaskets parked inside of the apartment complex garage and their car stared on fire. Luckily no one was killed.