I think opinions will vary widely on this, but here is my take.
I do not trim up my engine for trailering. Here are my reasons.
The main reason for trimming up your motor(s) for traveling is to assure that the skeg and bottom unit are not damaged. If you have enough ground clearance (say, 7" or more) then it is not necessary. I have a 10" clearance so I do not. If you have low clearance, then you need to trim up AND BRACE the motor to avoid damage.
Being extreme tight wads, most folks around here will trim up the motor a bit, stick a piece of 2X4 between the stern bracket and swivel assembly and then trim it back down until the board is firmly held in place. That raises the skeg up far enough and minimizes the pressure on the tilt trim mechanism.
A second reason often given for trailering in a tilt-up position is to prevent transom damage or fatigue. If you have enough clearance in the verticle position, then that doesn't make a lot of sense to me, unless you have a weak or very odd transom. The weight of the motor is the same, whether it is raised or not, so the vertical force on the transom remains unchanged. Raising the motor puts more shear (torque) pressure on the top of the transom. Thus, there is a market for various "transom savers" to transfer some of that shear force to the bottom of the transom, or the rear of the trailer (if you can find one long enough for that purpose and your trailer extends to or beyond the transom.) But for most boats and motors, I don't think they are necessary.
Whatever you do, do not trailer the motor in the raised position without some brace because that will potentially damage the tilt trim mechanism over the long run.