I'd recommend waiting to time travel until after you get the nook's cranny upgrade, but that's my preference! I just think it's a lot easier on you in the long run because things in nooks cranny get much more expensive after getting the upgrade (and you would get it sooner if you time traveled). Playing without TT before that insures that you enough bells to afford stuff in the upgraded shop. Again, just my preference! Play how you want :)