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 $109.34   8.492%   $118.6252   $118.63   $118.65   $118.60  
  Purchase #2 $250.73   5.594%   $264.7558   $264.76   $264.75   $264.80  
  Purchase #3 $6.36   4.477%   $6.6447   $6.64   $6.65   $6.60  
  Purchase #4 $161.63   3.678%   $167.5748   $167.57   $167.55   $167.60  
  Purchase #5 $235.33   3.388%   $243.303   $243.30   $243.30   $243.30  
  Purchase #6 $133.98   3.775%   $139.0377   $139.04   $139.05   $139.00  
  Purchase #7 $74.71   4.716%   $78.2333   $78.23   $78.25   $78.20  
  7 purchase totals = $972.08       $1,018.1745   $1,018.17   $1,018.20   $1,018.10