Codec selections for g.729 variants

[Dieselboy]

So I'm going through the INE CCNP Voice CIPT 1 video set and the instructor configures conferencing and transcoding on a Cisco 2800 router, to work with CUCM. He mentions to allow only these codecs:
codec g711alaw
codec g711ulaw
codec g729ar8
codec g729r8

By default, these are enabled:
codec g711ulaw
codec g711alaw
codec g729ar8
codec g729abr8
codec g729r8
codec g729br8

I have googled about the g729 and it seems that some g729 codecs are not interoperable, but I am finding conflicting information. Does anyone have any links to more information on this, or best practice?

I have a very large PVDM3 but I'm now configuring video conferencing and I want to reduce the number of allocated DSPs in use whilst ensuring compatability. I have followed the instructor but I would like to understand why this is done, exactly. 

[dlots]

Sadly I can't tell you to much about the 729 codec, but in general unless you have bandwidth issues your probably going to be using G.711ulaw (if your in the US or Japan) or G.711alaw (if your somewhere else, check with your local providers) as other compress data and can cause issues with fax machines or the phones def people use.

I will admit I am not much of a voice guy so if a voice guy disagrees with that please speak up.

[LynK]

We ONLY use:

G711ulaw
G729r8

[Dieselboy]

Cheers for your input