I would fix it immediately. It will get worse by usage and eventually
will become so much that you will be driving with a tail of blue smoke
behind your car. This is because the leaking oil will get on your hot
exhuast and burns. Think about the pollution you will cause to the
environment.

If you do not fix it you will have a mess under your car in no time at all.
The buildup of oil and road grime can be a fire hazard. I almost lost a
corvette once due to a grease fire that started close to the catalytic
converter. Luckily this happened in my driveway and I was able to extinguish
it right away. Ever since that experience I am paranoid about keeping the
underside of my cars relatively clean.
Peter

If it happened right after an oil change it might not be the seal.  If
the engine configuration is at all similar to mine (2.3L gas) then the
filter is on the top rear of the engine.

If the filter wasn't installed right (too loose, too tight, double
gasket, no gasket) then the oil might leak from there and drain down,
appearing to come from the rear main seal...

Since the rear main seal is a "normal" leak location even a good
mechanic might make this judgment in error.

Carefully investigate the upper half of the rear of the engine, and if
you are lucky you might find you don't need a rear main seal at all...

The reason this seal is expensive to replace is that the transmission
needs to come off the engine to get to it, which is a bit of labor...

Marty