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 $220.40   12.683%   $248.3533   $248.35   $248.35   $248.40  
  Purchase #2 $140.32   13.217%   $158.8661   $158.87   $158.85   $158.90  
  Purchase #3 $37.56   7.413%   $40.3443   $40.34   $40.35   $40.30  
  Purchase #4 $10.71   11.743%   $11.9677   $11.97   $11.95   $12.00  
  Purchase #5 $165.87   10.211%   $182.807   $182.81   $182.80   $182.80  
  Purchase #6 $102.28   6.383%   $108.8085   $108.81   $108.80   $108.80  
  Purchase #7 $222.66   7.368%   $239.0656   $239.07   $239.05   $239.10  
  7 purchase totals = $899.80       $990.2125   $990.22   $990.15   $990.30