If you're interested, there's a very spoilery explanation here: In fact, there is at least once case where that will be a bit confusing: the timelines of the two series remain mostly in sync with each other for the first 2/3 or the season, but sometime between episodes 9 and 15, Arrow pulls ahead of Flash.
you're watching on Netflix or DVD where jumping between shows is tedious) you don't need to watch the episodes interleaved get the full story.
However, if watching them in air-date order is difficult (e.g. So, watching in date-aired order is (almost) always going to work best for getting the entire story exactly as it was intended. crossovers that start on The Flash finish the next night on Arrow, or take place roughly between the previous and next episodes of the other show.) Otherwise, the crossover events all seem to fall roughly in line with the order the episodes aired (e.g. The first episodes of The Flash Season 1, and Arrow Season 3, take place at the same time, and there are cross-over cameos at the end of those episodes. The second season of Arrow contains what is essentially a two-part pilot episode for The Flash, where a mundane Barry Allen comes to Starling City as a forensics expert the end of that Arrow episode sets up the first episode of The Flash. In particular, watching in this order is the easiest way to guarantee not to spoil anything from either show by watching the other. There is only one time when the shows seem to be significantly out of sync, and it won't make a whole ton of difference. That is, if you want to get the most coherent story from both shows, you should watch Arrow up through the end of Season 2, then watch alternate episodes of The Flash Season 1 and Arrow Season 3. Ideally, you should watch them in the order they aired.