Shocks on our 7K lb TT made a HUGE improvement - less bouncing at the rear and better handling. Helped prevent things in our rear kitchen from bouncing around and doors opening in transit. I installed Monroe shocks on ours and Monroe says to install them at an angle.
Monroe, Mor-Ryde, LCI and Joyrider all want their shocks to be at an angle and compression/rebound rates are designed for being at an angle. It has more to do with shock travel and overall length, especially if mounting on the inboard side of the I-beams and if the axles are under-slung or over-slung. Ours are mounted between the tire and I-beams. I don't recall ever seeing photos with the shocks being vertical.
I highly recommend having shocks on a trailer. I also recommend Dexter EZ-flex equalizers and wet bolts.
First photo is a Lippert shock installation. Second photo is a Mor-Ryde installation. Third is a Joyrider installation. Fourth & fifth are photos of our Monroe setup. I had a set of tie plates modified to sandwich between our 3" axle tubes and springs for the lower mount. I had the upper mounting brackets welded to the I-beams by a certified welder at a frame shop. I needed the upper mounting points just above the bottom flange of the I-beams in order to clear the slide out shaft on the other side.