well, technically you don't have to, unless you remove the unit. my vts is still in my ski.
as far as adjusting on the fly vs adjusting after stopping is really a matter of personal preference, the manual trim certainly has its setbacks...
with vts if your cruising along at mid trim and a big 40ft cruiser comes along, you can pop the button and you've just moved your trim, you can adjust on the fly, with a manual I only move it when its a boring day and I decide to act like a donkey jumping around, otherwise its a hair off the middle all the time.
really that all depends on the type of rider, rec or hard core, if your hard core then ya, you need trim, if your the 95% of us average weekend dudes out having a good time then trim set just off the middle is just fine & dandy, and then a manual trim makes sense, MUCH less likely to break, so its a one & done job and one less thing that will anger you.