The set should always be put together so that the songs flow and there isn’t a lull in the set. Quite a common mistake is to put in a fast song, then a slow song, then a fast song, then a slow song. The idea behind this pattern is to avoid lumping a load of slow songs together. In practice all this actually does is make the faster upbeat songs come across with less impact, and the whole set never gets going.
As a FOH engineer you aren’t there to put the set together; that is really up to the band. But you should be aware of how the set works when it is played. If you think there is too much of a lull, or the energy of the set gets wasted too early, or doesn’t pick up fast enough, then consulting the band about any ...