High quality rosin allows for perfect bowing without scratchy sounds, especially if only a thin layer of rosin is applied. The general rule is:

• violinists and violists use a harder, dryer rosin
• cellists use a medium strong rosin
• bassists a softer rosin 

 Another important aspect for choosing the right rosin is the core material of the string that are being used:

• steel strings are easier played if you use a hard, dry rosin
• for synthetic, gut and gut wound strings use a softer rosin