Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $242.48   6.295%   $257.7441   $257.74   $257.75   $257.70  
  Purchase #2 $205.90   8.262%   $222.9115   $222.91   $222.90   $222.90  
  Purchase #3 $78.35   6.660%   $83.5681   $83.57   $83.55   $83.60  
  Purchase #4 $49.94   4.701%   $52.2877   $52.29   $52.30   $52.30  
  Purchase #5 $191.63   5.364%   $201.909   $201.91   $201.90   $201.90  
  Purchase #6 $214.96   8.749%   $233.7669   $233.77   $233.75   $233.80  
  Purchase #7 $6.88   5.328%   $7.2466   $7.25   $7.25   $7.20  
  7 purchase totals = $990.14       $1,059.4339   $1,059.44   $1,059.40   $1,059.40