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 $232.73   12.361%   $261.4978   $261.50   $261.50   $261.50  
  Purchase #2 $10.43   10.245%   $11.4986   $11.50   $11.50   $11.50  
  Purchase #3 $54.80   12.596%   $61.7026   $61.70   $61.70   $61.70  
  Purchase #4 $98.56   7.322%   $105.7766   $105.78   $105.80   $105.80  
  Purchase #5 $117.88   3.497%   $122.0023   $122.00   $122.00   $122.00  
  Purchase #6 $75.91   10.103%   $83.5792   $83.58   $83.60   $83.60  
  Purchase #7 $5.41   5.802%   $5.7239   $5.72   $5.70   $5.70  
  7 purchase totals = $595.72       $651.7810   $651.78   $651.80   $651.80