*update* 16/11/2017 - Hi, I have updated this solution with the remaster release. There is now a better video, a correct tracker in the video with names, correct numbers, 1080p60fps video and maybe a better english ;) The old solution is in the last spoiler if you want this solution.
There are 50 gold film reels in the game. None of them are missable and all gold film reels are immediately save after you pick it up. I recommend to collect the gold film reels in free roam mode, you can enter this mode after you have completed a desk (except patrol) from the main menu. Select cases then a completed desk and scroll down to "The streets of L.A.." This is the free roam mode.
If you want you can use the map and the tracker from Rockstar's socialclub.https://socialclub.rockstargames.com/games/lan/checklist
Under "Film Reels " you can see how many and which gold film reels you have already collected (map). The map from L.A. shows you also the locations but for many collectibles the map is really inaccurate. You should use the map then rather as a clue.
A Note, I collect in my video from north to south.