If I don't care about vertex count and I make texture map in substance painter that has tri-plane projection, is there any reason that I need to avoid UV seams as much as possible? And by the way,is UVs need to be flattened as much as possible if I use substance painter and have enough resolution for the map? Thanks.
This isn't correct. UV islands don't mean additional draw calls, but they do mean that the vertices are duplicated along the seam unless you use an unorthodox method for indexing vertices. You might also lose a bit of cache coherency on the GPU if you have too many UV islands, but you have to get to a point where you're…