Hello Ronald
In general, if you are using GRE, you do require 24 extra bytes minimum as you correctly mentioned. As a result, you can use an MTU of 1500-24=1476 as in the configuration of the lesson, and it would work. However, it is of benefit to use something like 1400 ip mtu with a tcp adjust-tss of 1360 because this will make the IP MTU much smaller than the interface MTU of 1500 (for Ethernet) and thus allow for any stray QinQ frames (+ 4 bytes) or further tunneling that may occur. The TCP adjustment of 1360 causes hosts transmitting over such a link to negotiate TCP sessions with an even smaller MTU so that no fragmentation will be necessary.
So if you are administrator of the whole network infrastructure and you are sure that the MTU settings you are applying are enough, then you should maximize the MTU sizes to what is necessary. If however your topology involves a WAN provider or a neighboring network for which you do not have full control over, it may be a good idea to use the 1400 values to ensure that no additional overhead will cause fragmentation or packet loss.
For more info on MTU, MSS, and related configurations, take a look at the following lesson:
I hope this has been helpful!
Laz