It's important that you loosen your bow after you play every single day-- "Good Night". If you leave your bow tight (the way you have it to practice) it will stretch out the hair and your bow hair will loose it's firmness and your sound will not sound good.
Over tightening a bow will cause the stick to warp and your sound will become bouncy in a bad way.
VIOLIN/VIOLA: If you can see a space in the middle of the bow between the stick and the hair that is just about enough room to fit a pinky finger -- it's just right!
CELLO/BASS: If you can see a space in the middle of the bow between the stick and the hair that is just about enough room to fit a thumb -- it's just right!