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 $6.50   9.559%   $7.1213   $7.12   $7.10   $7.10  
  Purchase #2 $70.35   12.604%   $79.2169   $79.22   $79.20   $79.20  
  Purchase #3 $132.71   3.270%   $137.0496   $137.05   $137.05   $137.00  
  Purchase #4 $74.40   12.251%   $83.5147   $83.51   $83.50   $83.50  
  Purchase #5 $65.66   12.228%   $73.6889   $73.69   $73.70   $73.70  
  Purchase #6 $163.90   7.759%   $176.617   $176.62   $176.60   $176.60  
  Purchase #7 $21.84   12.756%   $24.6259   $24.63   $24.65   $24.60  
  7 purchase totals = $535.36       $581.8343   $581.84   $581.80   $581.70