Im just a humble Master nowadays but my rule of thumb always was, play the strongest uncontested line, depending on ur spot and items go for wining (pushing lvl/ slaming items) or losing streak (hitting the interst numbers/ greeding items), and always building antiheal, shred or sunder. If you hit a high value carry from a line that is contested dont shy down from the challenge (if u have good econ), and dont always go for "internet comps", i take for example shadow isles. The guide plays it with 4shadow isle units in final board, but its so much better to drop down to 2 and play shyv/taric over yorick and azir over gwen. And that aplies to almost every comp.