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 $175.63   8.617%   $190.764   $190.76   $190.75   $190.80  
  Purchase #2 $70.70   3.647%   $73.2784   $73.28   $73.30   $73.30  
  Purchase #3 $229.54   7.593%   $246.969   $246.97   $246.95   $247.00  
  Purchase #4 $57.53   11.832%   $64.3369   $64.34   $64.35   $64.30  
  Purchase #5 $90.75   8.152%   $98.1479   $98.15   $98.15   $98.10  
  Purchase #6 $235.63   8.699%   $256.1275   $256.13   $256.15   $256.10  
  Purchase #7 $63.97   9.670%   $70.1559   $70.16   $70.15   $70.20  
  7 purchase totals = $923.75       $999.7796   $999.79   $999.80   $999.80