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 $18.70   7.803%   $20.1592   $20.16   $20.15   $20.20  
  Purchase #2 $13.83   4.330%   $14.4288   $14.43   $14.45   $14.40  
  Purchase #3 $172.70   8.283%   $187.0047   $187.00   $187.00   $187.00  
  Purchase #4 $115.38   5.709%   $121.967   $121.97   $121.95   $122.00  
  Purchase #5 $66.49   13.478%   $75.4515   $75.45   $75.45   $75.50  
  Purchase #6 $43.47   5.418%   $45.8252   $45.83   $45.85   $45.80  
  Purchase #7 $215.62   12.525%   $242.6264   $242.63   $242.65   $242.60  
  7 purchase totals = $646.19       $707.4628   $707.47   $707.50   $707.50