You misread the answer, and my suggestion wasn't terribly clear. Any imager should be "pro" associated (aka crap term "premultiplied") alpha.
There is a good reason, and I will expand this answer when I find some time later.
The bottom line is that only associated alpha models a real-world correlation, where unassociated doesn't.
Only associated alpha manages to model both occlusion and emission. Glows, convolutions, motion blurs, and a plethora of other transparent / translucent effects can only be modeled with associated alpha.
Think of alpha as selecting the over operation. The Alpha over node actually assumes associated alpha and performs the correct over operation, which is FG+(1-FG.a)BG.RGB.
Unassociated alpha can do none of these things. This is at least one of the reasons that PNGs are never used in a post production pipeline as they are incapable of using associated alpha. From the EXR Technical Introduction:
Calling the color channels “premultiplied” does not mean that the
color values in an image have actually been multiplied by alpha at
some point during the creation of the image, or that pixels with
zero alpha and non-zero color channels are illegal. Non-zero color
with zero alpha is legal; such a pixel represents an object that emits
light even though it is completely transparent, for example, a candle
flame or a lens flare.
In the visual effects industry premultiplied color channels
are the norm, and application software packages typically use
internal image representations that are also premultiplied.
From the actual TIFF specification, regarding TIFFTAG_EXTRASAMPLES tag that denotes what type of samples are stored:
The difference between associated alpha and unassociated alpha is not just a matter of taste or a matter of maths.
Associated alpha is generally interpreted as true transparency information.
Again, never, ever, use unassociated (crap term "straight" or "key") alpha, and instead associate it. This is what the unfortunately named "Convert Premul" does in the Alpha Over node. However, on direct Cycles renders, it is already associated and is never needed.
There is exactly one edge case that you must unassociate alpha before processing, and that is on colour corrections. To do so, you must be careful and divide all zero regions by a very small number as zero alpha with RGB nonzero is entirely valid, and represents an emission without occlusion.
The only time "Convert Premul" should ever be used is if the imager knows she is feeding it unassociated alpha, but like a good imager, she would never be using anything except the One True Alpha, associated alpha.
Zap Anderssen of Autodesk now:
Larry Gritz of OpenImageIO and Sony Pictures imaging team:
Jeremy Selan, two time Academy Award winning imager behind OpenColorIO, now at Valve:
The Infamous Adobe Photoshop thread. Pay key attention to the names, as there are many imaging legends in it, including Zap Anderssen dismantling one of the head developers of Photoshop, only to have Florian Kainz of ILM back up Zap's take on it:
EXR Technical Introduction:
I likely misspoke in the original question as, from memory, I can't remember exactly what that goofy keying node delivers in terms of alpha. I suspect it is unassociated, which makes the "Convert Premul" option work. Generally however, to avoid oopsies in the code etc., it is prudent to manually associate your alpha and control the merging to prevent data loss in the RGB channels. I still stand by the advice, however; always try to maintain a consistent state of alpha, and when you deviate, do so under scrutiny.