If the deck was 5 feet high, I would install the lattice verticaly. The reason being that I would not want any horizontal seams. The lattice is 4 feet wide so, every 4 feet there would be a vertical seam. I would use a vinyl "H" channel to cover the seam.
This is personal preference. I would prefer to see vertical lines to give the deck an appearance of height. This means that you will probably have to waste 3 feet of every section.
I would measure the width of the area, (or run), under the deck, determine the middle and decide if I was going to butt two 4 foot sections at the middle or start with the first section centerd. This would depend on how much area I would have left on the ends.
I would use aluminum, (or stainless), fasteners so there would be no rust.
Recap.. vinyl lattice, vinyl H connectors, and rust free fasteners = a rot free, stain free, and maintainace free job.
Almost anyone can build a deck but, as you have probably read in post by my compadres, proper planning, good materials, and pride, make the difference.
I hope this helped.
Now, where the advice comes from,
I was brought up in the Home Improvement trade and had my own company for quite some time until it was either that or my family.
I became a Building Inspector and now I'm a Zoning Officer and Housing Code Inspector. So, I have seen more ways to do things than the average contractor. Some were wrong, some were dangerous, and some were pretty scary!