i think 45min to an hour before the fireworks is a good rule of thumb. you may be able to get away with finding a spot in front of the castle about 30 minutes before the show, too. in december, we got a bench in front of the castle around 8:45pm or so (for the 9:25pm show)...got some hot cocoas and some treats for me, dh, and dd. we've only seen the fireworks show twice and both were in front of the castle, so i can't speak to viewing it from any other place. as for parades, we've only seen 1 parade and at the last-minute, swooped into an empty section in the roped off area on main street about 20 minutes before the parade started.
hopefully, others will chime in, too!