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 $6.73   10.566%   $7.4411   $7.44   $7.45   $7.40  
  Purchase #2 $117.28   10.178%   $129.2168   $129.22   $129.20   $129.20  
  Purchase #3 $62.62   5.630%   $66.1455   $66.15   $66.15   $66.10  
  Purchase #4 $152.42   4.254%   $158.9039   $158.90   $158.90   $158.90  
  Purchase #5 $168.62   9.819%   $185.1768   $185.18   $185.20   $185.20  
  Purchase #6 $40.97   4.265%   $42.7174   $42.72   $42.70   $42.70  
  Purchase #7 $231.63   6.333%   $246.2991   $246.30   $246.30   $246.30  
  7 purchase totals = $780.27       $835.9006   $835.91   $835.90   $835.80