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 $52.76   12.496%   $59.3529   $59.35   $59.35   $59.40  
  Purchase #2 $173.80   6.132%   $184.4574   $184.46   $184.45   $184.50  
  Purchase #3 $6.93   8.747%   $7.5362   $7.54   $7.55   $7.50  
  Purchase #4 $203.59   11.785%   $227.5831   $227.58   $227.60   $227.60  
  Purchase #5 $17.67   13.601%   $20.0733   $20.07   $20.05   $20.10  
  Purchase #6 $225.92   12.798%   $254.8332   $254.83   $254.85   $254.80  
  Purchase #7 $74.86   8.473%   $81.2029   $81.20   $81.20   $81.20  
  7 purchase totals = $755.53       $835.0390   $835.03   $835.05   $835.10