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 $218.87   13.527%   $248.4765   $248.48   $248.50   $248.50  
  Purchase #2 $71.68   4.528%   $74.9257   $74.93   $74.95   $74.90  
  Purchase #3 $48.39   12.153%   $54.2708   $54.27   $54.25   $54.30  
  Purchase #4 $104.26   5.366%   $109.8546   $109.85   $109.85   $109.90  
  Purchase #5 $217.94   9.562%   $238.7794   $238.78   $238.80   $238.80  
  Purchase #6 $62.97   12.325%   $70.7311   $70.73   $70.75   $70.70  
  Purchase #7 $163.30   4.671%   $170.9277   $170.93   $170.95   $170.90  
  7 purchase totals = $887.41       $967.9658   $967.97   $968.05   $968.00