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 $16.31   9.814%   $17.9107   $17.91   $17.90   $17.90  
  Purchase #2 $10.87   12.719%   $12.2526   $12.25   $12.25   $12.30  
  Purchase #3 $9.86   6.502%   $10.5011   $10.50   $10.50   $10.50  
  Purchase #4 $24.95   10.606%   $27.5962   $27.60   $27.60   $27.60  
  Purchase #5 $171.59   3.221%   $177.1169   $177.12   $177.10   $177.10  
  Purchase #6 $112.45   8.759%   $122.2995   $122.30   $122.30   $122.30  
  Purchase #7 $42.77   5.498%   $45.1215   $45.12   $45.10   $45.10  
  7 purchase totals = $388.80       $412.7985   $412.80   $412.75   $412.80