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 $130.82   5.621%   $138.1734   $138.17   $138.15   $138.20  
  Purchase #2 $24.32   11.241%   $27.0538   $27.05   $27.05   $27.10  
  Purchase #3 $59.34   12.198%   $66.5783   $66.58   $66.60   $66.60  
  Purchase #4 $69.89   12.448%   $78.5899   $78.59   $78.60   $78.60  
  Purchase #5 $236.82   6.422%   $252.0286   $252.03   $252.05   $252.00  
  Purchase #6 $119.67   9.458%   $130.9884   $130.99   $131.00   $131.00  
  Purchase #7 $122.32   13.281%   $138.5653   $138.57   $138.55   $138.60  
  7 purchase totals = $763.18       $831.9777   $831.98   $832.00   $832.10