I do tweak the levels on some of them before I put them in the polls, and sometimes do a little more later if needed, but I'm using GIMP now instead of Photoshop, and I find the color-correcting tools quite frustrating by comparison.
OT - I forgot to comment on this earlier. When I upgraded my computer, I could no longer activate my archaic version of Photoshop because the online activation for it no longer existed (I think it was CS1 - so really outdated, but still did everything I needed and cost me a ton of money at the time). A friend of mine suggested I try GIMP and being a software engineer, I am used to using freeware/shareware tools that can be a little difficult to navigate. However, I did not get along with GIMP at all.
I actually ended up downloading the free trial of Photoshop Elements and then later purchasing it after I verified it could do the things I needed. When I purchased Photoshop many years ago - Elements was crap they were giving away for free with any camera or scanner and it couldn't do much of anything. The current version can do almost everything my old version of Photoshop could do (or at least all of the features I used - things like levels and masks and effects and unsharpen mask and photo filters - it could even handle printer calibration profiles so that what you see on screen matches what prints from the printer). It took a while to figure out how to do some of these things as the interface is dumbed down - and sometimes it is a little clunkier, but if you know Photoshop, you can figure most of it out by tinkering and the rest out by Googling it. I think Amazon is currently selling the current version of elements for about $50 (not sure if that is a Black Friday sale or regular price).
So if you are frustrated with GIMP, you might want to try the free trial of Elements from Adobe's website and see if it meets your needs.