In any case put the pex in, you can always plumb or re-plumb what is feeding it, but you can't get the pex in later. Last summer when filling our pool I ran the water through the garage pex first. It warmed up the incoming water about 10F from 50F to 60F, cooled down the garage as well. I didn't want to try it with the basement because I was worried about condensation and it certainly did condense on the cold garage floor, but then again the house has AC and the garage doesn’t.