Here's a collection of my experience and practice:
1. EPDM on the lid o-ring. Best for keeping oxygen out and biggest surface area for oxygen ingress. However, I did have one keg that somehow had a slight bend on the lid and needed one of those oversized o-rings, which was silicone. I decided that was preferable to leaking gas. And then I just don't use that keg in circulation much.
2. EPDM on the liquid posts. Just because I had them. I don't do any crazy "change them every 6 months" schedule, but I have found recently that my liquid disconnects are super hard to put on and take off. They've been on about 2.5 years. So I guess change them every 2 years????
3. Silicone on the gas posts. I figured I didn't need both Liquid AND Gas posts to be a unique color, so I just went with Green on the Gas post. Too many times got the wrong one connected and really ticked me off. I am 100% satisfied with these green o-rings, even though they are silicone. I've had these probably ~7 years without leak problems or getting hard/sticky/swollen like the EPDM ones on my liquid lines.
4. EPDM on most of the dip tubes. Just because I had them. But I did find one keg that I couldn't get a good seal and had a leaker. Inspecting the o-ring found the base one on the keg was slightly larger OD. I had the keg since new, so it wasn't some wonky repair. So if replacing dip tube o-rings, hold on to your old one until you try the new one. I found most places don't specify the size of the dip tube o-rings, but other sites do, and there appear to be 2 sizes.