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 $19.98   4.360%   $20.8511   $20.85   $20.85   $20.90  
  Purchase #2 $232.87   10.520%   $257.3679   $257.37   $257.35   $257.40  
  Purchase #3 $129.57   3.574%   $134.2008   $134.20   $134.20   $134.20  
  Purchase #4 $67.77   6.463%   $72.15   $72.15   $72.15   $72.20  
  Purchase #5 $65.65   10.574%   $72.5918   $72.59   $72.60   $72.60  
  Purchase #6 $103.90   3.716%   $107.7609   $107.76   $107.75   $107.80  
  Purchase #7 $20.46   7.521%   $21.9988   $22.00   $22.00   $22.00  
  7 purchase totals = $640.20       $686.9213   $686.92   $686.90   $687.10