Hi, guys. I am currently rendering animation with settings - xvid, avi, h264. And for some reason i found out, that even if render has not finished yet you can go to the output folder and play what already been rendered. So i have a question, will it affect my animation, when it will be rendered, or i can play it with no fear?
You should never render directly to movie, go via images and assemble later.
I’ve learned this the hard way. There is a chance you will to 
Images can be freely viewed, but maybe better to clone them off to a laptop for viewing there (better speed).
As for movie, you can copy it when Blender has just started to render a new frame, and show that.
Things might get ugly if you open the file when Blender tries to save to it, depends on how Blender and viewing program work.