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 $21.40   10.485%   $23.6438   $23.64   $23.65   $23.60  
  Purchase #2 $112.96   12.507%   $127.0879   $127.09   $127.10   $127.10  
  Purchase #3 $56.33   3.509%   $58.3066   $58.31   $58.30   $58.30  
  Purchase #4 $203.33   13.720%   $231.2269   $231.23   $231.25   $231.20  
  Purchase #5 $215.54   7.239%   $231.1429   $231.14   $231.15   $231.10  
  Purchase #6 $247.35   6.515%   $263.4649   $263.46   $263.45   $263.50  
  Purchase #7 $43.65   3.581%   $45.2131   $45.21   $45.20   $45.20  
  7 purchase totals = $900.56       $980.0861   $980.08   $980.10   $980.00