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It's a common recommendation for 802.11a/g networks to disable lower rates to increase performance. Now, as it looks, the 802.11n (or 802.11-2012 clause 20, if you please) states that support for MCS0-7 is mandatory. All of them.
So, which one is true?
 1. The old recommendation is not valid for 802.11n networks
 2. I misinterpreted it and there's another way to selectively disable, say MCS0-4. Checked my AP - it's all or nothing (MCS0-7, MCS8-15, etc)

Rates configuration ability varies by vendor.  Some vendors will let you disable specific indices, limit spatial streams, and change the guard interval.  Others won't.

It's perfectly valid to disable support for some indices, but this might end up actually decreasing performance.  For instance, if a station has a strong signal but still has problems transmitting at a higher rate (e.g. temporary interference, buggy hardware, etc.) then it would get better throughput if it used a lower rate.

The better recommendation, if you can get away with it, is to disable non-802.11n rates, as they cause the stations in the BSS to use protection mechanisms when using newer modulation techniques.  This overhead is a performance killer.  Many networks require support for older wireless stations, so it can be difficult to abandon support for 802.11b/g.

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@@ron

Hi, thanks for replying.
I know of protection modes and various vendor capabilities. However, this does not answer my question: is it true that as per current standard you MUST treat MSC0-7 as a single block of rates and MUST have them enabled at all times? Or, I just did not interpret it properly, and you must IMPLEMENT MSC0-7, but are allowed to turn them on/off selectively?

Look at 20.3.5: "MCS 0 to 7 are mandatory in 20 MHz with 800 ns GI at all STAs."
Also take alook at B.4.19.2 (HT PHY features), where the above MCS are listed as mandatory.

I see nothing that makes me think it must be treated as a single block which is always enabled.

With 802.11g, 6, 12, and 24 Mbps were also described as mandatory, but every AP I ever saw let you disable them one way or another. 

The second section you reference B.4.19.2, is the PIX Proforma (sometimes called the manufacturers questionaire).   And the sections   HTP2.3.1.1 thru HTP2.3.1.8 covering MCS 0 - MCS 7 are clearly shown as separate features.