Reusing oil containers I generally buy large containers of oil (olive, canola, peanut, etc.) and just refill a handful of smaller oil bottles.  That way it's cheaper in the long run, and the smaller bottles are easier to deal with.

I generally just add more oil when a bottle starts getting low.  I imagine this could have some bad effects:


Oil remaining in bottle could go rancid (though it's getting diluted each time, so I imagine very little remains over time)
If any bacteria/contaminants get it, they'll just remain in there forever.


Are these realistic concerns?  Are there things that could start growing in pure oil?  Would it be worth the hassle to periodically let the bottles run empty and wash them out / replace them?
Yes these are realistic concerns.


the tiniest amount of rancid oil can make your dish taste bad.
mold can grow on fat (e.g. penicillium in cheese).
oil only has residual amount of water, so it will not accelerate most bacterial growth.
shelf life for oil is about 6 months to a year in a dark cool cupboard
shelf life is about a year and more in fridge (but the oil clouds and solidifies). 


my suggestion would be that when your container is empty, just wash it with a natural degreaser (e.g. vinegar) instead of topping it off. Or top it off, but wash the smaller bottles every 6 months.

UPDATE

Be very careful about what kind of oil you use, in particular if it is not commercial-grade. In this other question you will see that oil or other canned products that provide anaerobic environment (=without oxygen) can be good conditions for some bacteria. Some of them are specifically bad. See also the wiki page for the C. Botulinum for an example.