If they smell bad or odd, then surely toss them out.
White & brownish eggs are easier to see thru, where blue, green or very dark brown eggs (like from Marans chickens) are very difficult to candle & see thru. So, take that into consideration if it applies.
I candle on day 7 (or 10 is easier to see the "dark mass" chick with a weaker flashlight), then on day 14 & on day 18 (which is the last chance before "lockdown"). I sometimes mark an "x" on the wide end of the egg, if they are "iffy's" (in other words if I suspect they are not good/fertile). Then on day 18 I make a final decission on if it's time to toss them; if I still can't be sure & decide to leave it in during "lockdown," then always give it the sniff test to toss any that smell odd.
It's also interesting to look for the air cell at the wide end, the dark mass in the middle area & the clear egg white at the pointed end. My old 9 LED 4" flashlight was good enough to see those 3 areas, but my new NEBO 4" flashlight shows chick movement, even more interesting.
The best way that I've learned from "iffy" or clear eggs being tossed, is to open them over a paper towel in the trash can. That way I confirm what I've seen or learn about what I wasn't sure about. By opening eggs that are being tossed, I've learned not to be hasty & open a fertile egg by mistake.
Happy candling & hatching!