TLDR - You want that airgap transferring oxygen via steadily replenished atmo-gasses into the soil as much, and as low, as possible. 
The holes are there so the atmo-gasses in the airgap can pass into the soil, which is why the airgap's there if you think about it, well, that and to limit the water transfer rate into the planter - but those two results live on the same axis.
Note also that there doesn't seem to be a top-end to how much atmo-gas you can pass into the airgap itself and subsequently the soil, without negative response, provided your wicking rate is good. I've tested this idea to death (er, life, I meant life!). To that end, a second drain hole on the other side of the pot will help and not hurt, and will get more atmo-gasses in into the rez and subsequently up through the holes in your upside-down bucket rez. I hope that's clearish.
I tested the "can there be too much incoming atmo-gas to the reservoir and subsequently the soil", question in multiple ways. During testing I ppm'd soil, ph'd rez and soil, then inspected roots carefully at the end. For testing I created a rez with more airgap-level holes than reservoir walls, it had almost nothing but holes around the entire pot for full circumfluence and 4 inch deep, as the airgap was min. 4 inches deep. Testing, yield, and root exam were all nominal.
I've also hammered a rez, full-time, with airstones; two 5 inch dia. circular stones running off a 30w air pump running full-time for an entire grow, also with a large airgap and this time with few escapes for the air as pressure builds except for up through the soil (I omitted drain holes and used floating level marker). Results: Nominal - except for the fact that the rez water became so oxygenated that it filled up with DWC-style herring-bone roots. So I had a DWC and a SIP going, on the same plant! Topside, the plant's yield and quality were very similar if not identical to its clone, run in a traditional SIP.
Drilling lots of little holes into the "reservoir body" (the upside down bucket), on its top as least as far down as the reservoir airgap goes, permits that air to enter the soil matrix at the bottom where it is needed to oxygenize the watery bottom layers of soil. The word 'aerate' here isn't specific enough, as the concept is oxygen levels which the plants themselves will remove from the water, without taking the water, which can cause root rot, just like DWC. So, holes at the top are great, but since that gap often becomes much wider with rez level dropping everyday, holes all the way down the sides mean intermittent aeration and oxygen to those low-low levels where root rot is likeliest. Just make them small enough so wet soil can't easily fall through, and then, so long as you don't topwater so much as to send that soil through the wee holes, you can avoid covering it with a porous, synthetic fabric barrier. 1/8 inch hole max for me does the job whether with peat or LOS/TLO, so I think 1/8 dia. hole-size is OK to use with any matrix. Landscaping cloth, the cheap two-year rated stuff works great if truly needed, or if you're nervous and want to play it as safe as you can dream it. Be aware that cotton t-shirts, burlap, indeed every natural material I've tried, tight weave or loose, thick or thin, all rot out within 6-8 weeks. Felt lasted the longest but did not even last half a grow.
I wouldn't blame anyone for questioning how the hell I actually know the timetable for this disintegration... after I discovered the issue I began tests with no plants, just soil.
I'm constantly working to discover SIPs' actual limits, blind-spots, and with that data try to design some capabilities that might introduce a touch more controllability for us. Not that I'm having issues. I muck around this way because after forced retirement in my early forties, I desperately need intellectual and physical challenges, and I may have a slightly stronger-than-is-healthy need to feel useful.
). I don't believe I had any bad effects due to that rim being on the res. You can see roots made it all the way down to the bottom of the bucket between the walls of the bucket and the res.
for sharing that!
