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 $79.85   12.828%   $90.0932   $90.09   $90.10   $90.10  
  Purchase #2 $94.63   11.801%   $105.7973   $105.80   $105.80   $105.80  
  Purchase #3 $79.37   8.617%   $86.2093   $86.21   $86.20   $86.20  
  Purchase #4 $28.34   3.790%   $29.4141   $29.41   $29.40   $29.40  
  Purchase #5 $144.70   7.547%   $155.6205   $155.62   $155.60   $155.60  
  Purchase #6 $59.98   10.826%   $66.4734   $66.47   $66.45   $66.50  
  Purchase #7 $80.56   5.399%   $84.9094   $84.91   $84.90   $84.90  
  7 purchase totals = $567.43       $618.5172   $618.51   $618.45   $618.50