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 $8.89   12.430%   $9.995   $10.00   $10.00   $10.00  
  Purchase #2 $109.37   13.767%   $124.427   $124.43   $124.45   $124.40  
  Purchase #3 $225.45   7.480%   $242.3137   $242.31   $242.30   $242.30  
  Purchase #4 $135.41   9.812%   $148.6964   $148.70   $148.70   $148.70  
  Purchase #5 $27.62   11.814%   $30.883   $30.88   $30.90   $30.90  
  Purchase #6 $33.71   12.862%   $38.0458   $38.05   $38.05   $38.00  
  Purchase #7 $15.32   11.708%   $17.1137   $17.11   $17.10   $17.10  
  7 purchase totals = $555.77       $611.4746   $611.48   $611.50   $611.40