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 $120.58   13.621%   $137.0042   $137.00   $137.00   $137.00  
  Purchase #2 $241.34   11.419%   $268.8986   $268.90   $268.90   $268.90  
  Purchase #3 $140.25   9.779%   $153.965   $153.97   $153.95   $154.00  
  Purchase #4 $56.85   11.856%   $63.5901   $63.59   $63.60   $63.60  
  Purchase #5 $51.61   13.679%   $58.6697   $58.67   $58.65   $58.70  
  Purchase #6 $245.99   6.295%   $261.4751   $261.48   $261.50   $261.50  
  Purchase #7 $48.34   9.528%   $52.9458   $52.95   $52.95   $52.90  
  7 purchase totals = $904.96       $996.5485   $996.56   $996.55   $996.60