before moving forward, we should decide what kind of extensions are a no-no.
are r3 ok? because it can be considered a compilation, one song after another.
another example are short ~29 second anime opening/ending songs, like Harumachi clover, where the song is under 30 seconds but mappers have been extending them to hit the 30 second mark.
as for cuts, i personally cut the song for playability and "not making the player bored" sake. Most songs have pretty much copy pasted rhythm.
The most common song structure is intro, verse, pre-chorus, chorus (or refrain), verse, pre-chorus, chorus, bridge ("middle eight"), verse, chorus and outro, and then the song is pretty much over. pretty much copy paste with different vocals.
making a song shorter serves more purpose than just putting less work. in fact, it does not require that much work if i don't cut a song, i just copy paste items, change them up a little and etc.