[Invincible]

Copper sulfate is commonly available in its pentahydrate form, which has a deep blue color. In preparing solution, it is much easily dissolved in solution, however it is also efflorescent.  Depending on storage conditions it can be partially dehydrated.

Some chemicals can have multiple hydrates, so even if it is weighed accurately, if the hydration state is not consistent, its not possible to properly measure out the desired amount of the compound.

If you need a very accurate solution, do you start off with oven drying and start from anhydrous form or what?

[Borek]

If you need very accurate solution you prepare it as approximate, then you use some analytical method to standardize the solution. That's quite common approach, copper sulfate is not unique.

A liitle bit more here:

http://www.titrations.info/titration-standard-substances

Page is about titration, but the same approach is used in analytical chemistry in general.