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 $49.98   11.228%   $55.5918   $55.59   $55.60   $55.60  
  Purchase #2 $166.50   10.269%   $183.5979   $183.60   $183.60   $183.60  
  Purchase #3 $170.78   3.851%   $177.3567   $177.36   $177.35   $177.40  
  Purchase #4 $244.57   12.726%   $275.694   $275.69   $275.70   $275.70  
  Purchase #5 $145.50   11.242%   $161.8571   $161.86   $161.85   $161.90  
  Purchase #6 $235.98   10.842%   $261.565   $261.57   $261.55   $261.60  
  Purchase #7 $124.32   9.775%   $136.4723   $136.47   $136.45   $136.50  
  7 purchase totals = $1,137.63       $1,252.1348   $1,252.14   $1,252.10   $1,252.30