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 $53.25   10.216%   $58.69   $58.69   $58.70   $58.70  
  Purchase #2 $245.51   12.746%   $276.8027   $276.80   $276.80   $276.80  
  Purchase #3 $30.37   7.247%   $32.5709   $32.57   $32.55   $32.60  
  Purchase #4 $78.99   4.479%   $82.528   $82.53   $82.55   $82.50  
  Purchase #5 $64.79   6.126%   $68.759   $68.76   $68.75   $68.80  
  Purchase #6 $205.94   4.462%   $215.129   $215.13   $215.15   $215.10  
  Purchase #7 $99.39   5.242%   $104.6   $104.60   $104.60   $104.60  
  7 purchase totals = $778.24       $839.0796   $839.08   $839.10   $839.10