Kick panels would be my ideal place, in my install. As i know they work pretty well on axis, it would be strange if they didn't, Majority of the foreigners that visit DIYMA (i forget which countries directly) use them mounted in there dash's, some kicks so i'm sure there getting reflections off the dash, which in a way would be on axis...Mabey?
The Triuses are built to excel Off-axis, to allow for a lot of different placement constrains. They do not have the same dispersion character as most drivers, which is purposely so. They do really well in most stock dash speaker locations. That means that their upper range (5 k up) is a bit lifted if you use them on-axis. (check Npangs measurements).
I have designed a simple notch filter that takes care of most of the tonal issues, and with that it works nice on axis to IMO, but for best results, as always aim-listen-repeat is what goes.