Why it is important to clearly estimate point size of user stories. It is important because if we are over estimate the user stories point it will give a bad result on velocity and burdown chart. Over estimate is have to but if we are over2 estimate, that have to avoid. When we over estimate to some of user stories then it will cost of free time and this will cause of increasing of budget.
Under estimate is also bad on estimating user stories point... because if we under estimate a user story point... it will make delivery delay and this will dissapointed user because its not follow the schedule and this will make corporate to think about your position of Project Manager and Developer. under estimate user story point have to be avoid.
Estimating a user story point is okey if you over estimate it a little but avoid under estimate a point. As an example, you have a user story about "As a user i could search an item in the system". And you have estimate it that it will cause about 4 point however the actual point is 3. It is okey if you over estimate a little but if you estimate the user story is 2 point... it can cause delay and developer have to give a very high commitment to the user story because you have under estimate the user story.
Estimating actually is quite easy. On all user story you have create... chose the easiest one which cause 1 point. and choose other user story. If the user story is twice difficult to the first one, so multiply it. so the second user story will be 2 and so on. And on estimating user story point, you have sit down with all you developer and pruduct owner and project manager. So estimating point to user story will be accurate.
Adios