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 $51.82   12.853%   $58.4804   $58.48   $58.50   $58.50  
  Purchase #2 $158.78   11.695%   $177.3493   $177.35   $177.35   $177.30  
  Purchase #3 $180.72   11.284%   $201.1124   $201.11   $201.10   $201.10  
  Purchase #4 $236.62   9.615%   $259.371   $259.37   $259.35   $259.40  
  Purchase #5 $46.96   12.552%   $52.8544   $52.85   $52.85   $52.90  
  Purchase #6 $97.30   8.637%   $105.7038   $105.70   $105.70   $105.70  
  Purchase #7 $21.80   12.412%   $24.5058   $24.51   $24.50   $24.50  
  7 purchase totals = $794.00       $879.3771   $879.37   $879.35   $879.40