On any kind of bar it's good to always tighten the tape in the same direction your hands naturally rotate. This creates a wrap that tends to be tightened rather than loosened by your hands' natural motion. On bullhorns and bar tops, your hands rotate across the top and towards the outside, so wrap em that way.

Whatever works though, if your wrap is nice and tight it probably won't come loose either way. Just try not to have both sides going the same direction, cuz it looks incorrect: i.e.

not like this:

\ ..... \
\ .....\
\ .....\

or this:

/....../
/....../
/....../

but like this:

\ ..... /
\ ...../
\ ...../

or this:

/......\
/......\
/......\