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 $231.90   12.531%   $260.9594   $260.96   $260.95   $261.00  
  Purchase #2 $74.31   8.158%   $80.3722   $80.37   $80.35   $80.40  
  Purchase #3 $52.56   11.803%   $58.7637   $58.76   $58.75   $58.80  
  Purchase #4 $206.51   3.529%   $213.7977   $213.80   $213.80   $213.80  
  Purchase #5 $242.41   6.396%   $257.9145   $257.91   $257.90   $257.90  
  Purchase #6 $5.50   3.577%   $5.6967   $5.70   $5.70   $5.70  
  Purchase #7 $51.65   13.695%   $58.7235   $58.72   $58.70   $58.70  
  7 purchase totals = $864.84       $936.2277   $936.22   $936.15   $936.30