You're not doing anything wrong by doing it that way - and it will take a long time (if ever) for the trampoline to fray and tear there, as long as there are no spurs or snags on the aluminum track. It's mostly cosmetic.
You can try this method to minimize it:
1) Install the trampoline in the tracks, getting it as far into that corner as you can by hand.
2) Tighten the outer 2 or 3 grommets worth of each side's rear lacings to "set" the corner.
3) Tighten the center lacing, but go easy on the first couple of grommets - just enough to pull the wrinkles out. When you get to the part that's supported by the track, crank on it.
4) Tighten the rear lacings - crank on them.
5) Take another pass down the middle.
My guess is that even after all this - you'll still eventually end up with the same look after it's been sailed a while and things have settled in.
Or, you could do what's done on the 17:
There's a fairlead bolted into the crossbar track at the corner, and a grommet in the corner of the tramp. Some people shackle the tramp to the fairlead, but I use a couple lashings of low stretch, small diameter line (2 mm Amsteel). It's kind of the ultimate solution.