After reading up on Front Height at Dolby, http://www.dolby.com/us/en/professional/technology/home-theater/dolby-pro-logic-iiz.html
, I would say direct firing are ideal if you don't have rear surrounds, but bipoles could be better if you do have rear surrounds.
Some key things I read were:
"Because it processes only nondirectional sounds for the height channels, ..."
"Supported upmixed output configurations based on 5.1 sources with independent Ls and Rs channels:
-5.1 to 7.1 Height [Ls, Rs to Ls, Rs, Lvh, and Rvh]
-5.1 to 9.1 [Ls, Rs to Ls, Rs, Lrs, Rrs, Lvh, and Rvh] "
"... 7.1 Back sources and offer the following upmixed output configurations:
-7.1 Back to 7.1 Height [Ls, Rs, Lrs, and Rrs to Ls, Rs, Lvh, and Rvh]
-7.1 Back to 9.1 [Ls, Rs to Ls, Rs, Lrs, Rrs, Lvh, and Rvh]"
For 5.1 and 7.1 sources "... L, C, R, and LFE are bypassed."
This leads me to think that the only sounds output to the Front Height Channels are sounds that are being played simulaneously on both Side Surround Channels for 5.1 or 7.1 sources, but if your setup lacks Rear Surrounds and you have a 7.1 source then Rear Surround information will also be used to produce the Front Height channel . It does also mix to them from stereo, probably using the same idea but pulling from the upmixed side surround channels.
So with no rear surrounds direct firing bookshelfs aimed straight back above the viewers heads might be best since you may have sounds that are actually supposed to be behind you (though I can't say for sure without knowing more exact details about the upmixing). If your setup has rear surrounds then bipoles aimed above the viewers heads would create a more spacious sound while still maintaining the height (with the drivers aimed apart horizontally not vertically, obviously).
A 180 degree arranged bipole or dipole type speaker would probably be a bad choice since firing into the front wall may keep the sound farther forward rather than making it come from above your head and might lessen the perceived height as well.
I just upgraded my side surrounds and don't have the room to add rear surrounds so I'm going to try out Front Heights myself. I will be using small direct firing satellite speakers and I plan to aim them straight back over my listening position, mounted above my front L/R speakers, about a foot or two below the ceiling.