you can make them do whatever you want them to do. So yes you can create a pipe that follows the contours of the grade. In order to do this you will either have reapply a rule set to each individual piece or put them in a profile and move the pipe around in that.
I know several people that would tell you to use a structure rather than a null structure, but to make inserting then trimming to a block for bends, devices, etc. easier I would use a null. The reason why is because a null structure uses a different insertion then the rest of the structures making it easier to insert a block where you want it rather than figuring offset from a sump or rim.
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