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 $102.49   6.269%   $108.9151   $108.92   $108.90   $108.90  
  Purchase #2 $136.29   8.800%   $148.2835   $148.28   $148.30   $148.30  
  Purchase #3 $156.36   13.376%   $177.2747   $177.27   $177.25   $177.30  
  Purchase #4 $26.39   6.107%   $28.0016   $28.00   $28.00   $28.00  
  Purchase #5 $5.98   3.480%   $6.1881   $6.19   $6.20   $6.20  
  Purchase #6 $136.66   7.343%   $146.6949   $146.69   $146.70   $146.70  
  Purchase #7 $25.59   9.184%   $27.9402   $27.94   $27.95   $27.90  
  7 purchase totals = $589.76       $643.2981   $643.29   $643.30   $643.30