It depends on what you are using this for - link capacity or QoS settings ("priority" and "bandwidth" statements in the router).
For capacity calculations on WAN links, you need to take into account the full overhead. This includes the flags that separate the frame relay frames and the FCS. So a full frame bandwidth overhead is 7 bytes (you only count 1 flag) For LAN line capacity, you also need to take into account the FCS, the preamble and start of frame delimiter, which is another 8 bytes on top of your 18 (or 22 in the case of 802.1Q)
For QoS settings, you disregard the FCS/CRC bytes. This is because these are normally stripped off in the interface hardware and are therefore not seen by IOS. This means that the QOS design guide is right for frame relay (4 bytes). However, most ethernet access is 802.1Q nowadays which requires a layer 2 overhead of an extra 4 bytes on top of the standard 14 bytes ethernet header, giving a total of 18 bytes.
ATM is an interesting case, where the QoS mechanisms look at the AAL5 level, with just 8 bytes overhead per packet. The actual line overhead with ATM is much greater.
So they are all sort of right in their own way! It depends what question you are answering.
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