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 $179.81   13.331%   $203.7805   $203.78   $203.80   $203.80  
  Purchase #2 $113.86   9.212%   $124.3488   $124.35   $124.35   $124.30  
  Purchase #3 $125.33   4.429%   $130.8809   $130.88   $130.90   $130.90  
  Purchase #4 $185.63   10.870%   $205.808   $205.81   $205.80   $205.80  
  Purchase #5 $22.71   4.420%   $23.7138   $23.71   $23.70   $23.70  
  Purchase #6 $114.51   10.119%   $126.0973   $126.10   $126.10   $126.10  
  Purchase #7 $178.44   5.849%   $188.877   $188.88   $188.90   $188.90  
  7 purchase totals = $920.29       $1,003.5063   $1,003.51   $1,003.55   $1,003.50