I think lower than six feet will not be good since the plants when cascading make very long branches , I mean u could always prune it , but the plants will be happier if u leave them get more plant mass to them . I am planning on having my posts as heigh as me ,maybe an idea shorter at around 160- 170 cm, since they cascade and u need to have space below the top of the post.
Now for the top of the post, for me it is needed since it spreads the weight of the plant mass and eases formation of the canopy of the whole thing.
I dont think the top couses the post to fall or tip since without it the plant will get similarly top heavy and without a sturdy base it would also fail to support itself.... .
U just need to make a sturdy base, if its cement pole like In my situation , just bury it deep enough, mine is tall around 225 cm and I will have around 70 cm from the pole, burried into the soil