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 $242.89   11.155%   $269.9844   $269.98   $270.00   $270.00  
  Purchase #2 $33.36   12.189%   $37.4263   $37.43   $37.45   $37.40  
  Purchase #3 $237.51   6.695%   $253.4113   $253.41   $253.40   $253.40  
  Purchase #4 $44.94   11.821%   $50.2524   $50.25   $50.25   $50.30  
  Purchase #5 $67.35   8.650%   $73.1758   $73.18   $73.20   $73.20  
  Purchase #6 $196.70   4.712%   $205.9685   $205.97   $205.95   $206.00  
  Purchase #7 $116.86   10.819%   $129.5031   $129.50   $129.50   $129.50  
  7 purchase totals = $939.61       $1,019.7218   $1,019.72   $1,019.75   $1,019.80