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 $246.76   11.654%   $275.5174   $275.52   $275.50   $275.50  
  Purchase #2 $100.34   13.182%   $113.5668   $113.57   $113.55   $113.60  
  Purchase #3 $116.86   7.166%   $125.2342   $125.23   $125.25   $125.20  
  Purchase #4 $209.42   11.612%   $233.7379   $233.74   $233.75   $233.70  
  Purchase #5 $12.69   7.515%   $13.6437   $13.64   $13.65   $13.60  
  Purchase #6 $53.99   4.397%   $56.3639   $56.36   $56.35   $56.40  
  Purchase #7 $15.50   5.258%   $16.315   $16.32   $16.30   $16.30  
  7 purchase totals = $755.56       $834.3789   $834.38   $834.35   $834.30