Pray that it doesn't rain.
Seriously though, rain will definitely affect your plans and photos. I was at the Kurama Fire Festival 2 years ago and the rain made it so smoky! It was hard to take pictures while holding an umbrella too. But I guess the rain reduced the number of visitors so it balanced out in the end. Expect lots of fire and bare-bottomed guys. It will be an eyeful.
I've never been to the Jidai Matsuri (been to the other 2 only), but I've heard of people who recommends the Sanjo-dori near Sanjo Keihan station if you want to take pictures. Of course, expect photographers to be there at least a couple of hours before the parade starts. If you don't want to spend the whole afternoon watching it, head to the Imperial Palace early. They parade through lunch so bring some snacks if you're planning to sit through it.
If there's no rain and you want good spots for the Kurama festival, head straight up north with food and drink once you're done with the Jidai Matsuri because everyone else will be heading the same way. Be ready to stay late 'cos the action happens at the temple at 8pm. After that, everyone will head to the station together so expect a long wait for trains.
Good luck and have fun!