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 $72.72   12.264%   $81.6384   $81.64   $81.65   $81.60  
  Purchase #2 $233.61   12.869%   $263.6733   $263.67   $263.65   $263.70  
  Purchase #3 $121.83   4.533%   $127.3526   $127.35   $127.35   $127.40  
  Purchase #4 $24.25   12.567%   $27.2975   $27.30   $27.30   $27.30  
  Purchase #5 $248.97   5.244%   $262.026   $262.03   $262.05   $262.00  
  Purchase #6 $5.94   3.427%   $6.1436   $6.14   $6.15   $6.10  
  Purchase #7 $59.52   6.550%   $63.4186   $63.42   $63.40   $63.40  
  7 purchase totals = $766.84       $831.5500   $831.55   $831.55   $831.50