The above diagram is a cutaway of a single coil cartomizer to display its anatomy and to show liquid (red arrows) and air flow (blue).
The action of drawing, pulling, or puffing pulls juice not only through the hole in the carto from tank, but also from the top of the filler in the carto. This is why after vaping for a few minutes the top of the filler seems to appear dry. Under normal situations the filler doesn't wick completely to the top of the carto from the lower holes; this is normal and doesn't matter. What does matter is to keep the filler surrounding the heating coil(s) wet with liquid to prevent burning.
Since the heating element in a single coil carto is near the bottom of the carto, one or two holes in the lower third of the casing should be perfectly adequate for most e-liquids.
It is with dual coils where I have had issues, and I have stopped using them. The second coil is located in the middle third of the carto. Since the juice level in a carto will fall due to suction while vaping, there's a likelihood that the top coil may not get enough liquid around it and burn its covering or surrounding filler, leading to a burnt taste. And we all know it's impossible to get rid of that burnt taste once it happens. That carto needs to be trashed and replaced.
I know there are people who place multiple holes in their cartos, including ones in the middle or upper third of the casing, and they get decent results. Where I see a problem arising with this method is as the level of the liquid falls in the tank, the upper hole(s) become exposed to air. A cartotank functions because of negative pressure in the tank -- both liquid and air. When you add a hole in the upper third of the casing and it becomes exposed to the air in the tank, the negative pressure in the tank is disrupted as air is allowed to leave the tank through the carto.
Perhaps this is a more complicated explanation than required, but long story short I don't think its a good idea to have holes in the middle or upper third of the carto, unless you are going to top off your tank before any holes are exposed. Your results may and likely will vary from mine.
Just as a burnt carto is ruined for life, a flooded carto is also doomed for the most part. Once the fluid barrier has been breeched, even after blowing out the center hole, it will never work as well as before.
___
I'm a firm believer that cartotanks require a specific type of draw to work properly. Too hard of a pull forces too much liquid into the carto and can lead to flooding issues. A cartotank needs a minimum of time to rewick itself. A slow - deliberate - gentle - patient - draw of 3 - 6 seconds seems to work best.
___
I stopped punching my own holes in my cartos a long time ago. I never found a punch tool that didn't deform the casing from round to oval at least a little. This deformity causes unneccesary wear and tear on the o-rings in the end caps each time the carto is moved through them. Of course, it is the o-rings which hold the tank snuggly on the cartomizer, and prevents the tank from leaking. They are very important. Using laser-drilled or pre-punched cartos eliminate this issue, and in my experience they wick juice as well as tool-punched cartos.
While we are on the topic of o-rings, I think it is important to mention using a tank accessory tool. The best one is the Slap Yo Moma tool from Big Daddy Vapor. Not only does it make filling and assembling a cartotank much easier and faster, it will protect the o-rings from the sharp top edges of a cartomizer which can cut pieces away when you pass it through the end caps.