If you are using album-based ReplayGain, you are very unlikely to run into an album with less than -6dB of correction, and less than 6dB of headroom on the tracks. Even less likely with 4dB.
If you're using track-based ReplayGain though, I would avoid making any adjustment.
That being said, I do have at least one album with very high fidelity recordings where the album gain is -2.2dB, and four of the sixteen tracks have less than 2dB of headroom. So even if you were to only run a +4dB adjustment, you would run into clipping. If you were to run a +6dB adjustment, you would clip about half of them.
The "Automatic based on current playlist" option seems to negate the purpose of ReplayGain. It will only make your current playlist sound the same level, and if you then play something else, it will not match. The whole point of ReplainGain is to keep everything at the same relative volume.
Even undithered 16-bit audio has more than enough dynamic range for ReplayGain to work correctly, so I just use album-based gain with no adjustment, and turn up the volume on my amplifier.
I do wish that we were able to view the album gain from within MC18 though (it only lists track gain) and that we had some more control over analyze audio. I previously had everything tagged through dBpoweramp when ripping my CD library, but had to overwrite that in MC18 if I wanted Intensity (a crude dynamic range measurement) and BPM in the library. Unfortunately I have a couple of albums where there are multiple discs, and each disc has its own name. So I would want them to all have the same album value in ReplayGain, but MC18 only seems to check if the "album" tag is the same.
I've set it to fixed +4dB but on some files (with very low RG values = lower than 4 ) this results in slight clipping. So I need to keep an eye on the RG values on what I'm playing.
This will only result on clipping if the ReplayGain value is less than 4dB
and there's less than 4dB of headroom in the recording. This is fairly uncommon.