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 $241.59   12.544%   $271.895   $271.90   $271.90   $271.90  
  Purchase #2 $181.54   10.592%   $200.7687   $200.77   $200.75   $200.80  
  Purchase #3 $51.81   5.693%   $54.7595   $54.76   $54.75   $54.80  
  Purchase #4 $107.69   8.687%   $117.045   $117.05   $117.05   $117.00  
  Purchase #5 $58.74   13.340%   $66.5759   $66.58   $66.60   $66.60  
  Purchase #6 $113.33   5.188%   $119.2096   $119.21   $119.20   $119.20  
  Purchase #7 $143.60   5.646%   $151.7077   $151.71   $151.70   $151.70  
  7 purchase totals = $898.30       $981.9614   $981.98   $981.95   $982.00