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 $159.84   3.552%   $165.5175   $165.52   $165.50   $165.50  
  Purchase #2 $141.63   10.798%   $156.9232   $156.92   $156.90   $156.90  
  Purchase #3 $15.69   4.336%   $16.3703   $16.37   $16.35   $16.40  
  Purchase #4 $7.41   7.828%   $7.9901   $7.99   $8.00   $8.00  
  Purchase #5 $10.40   9.519%   $11.39   $11.39   $11.40   $11.40  
  Purchase #6 $150.89   5.473%   $159.1482   $159.15   $159.15   $159.10  
  Purchase #7 $157.94   9.384%   $172.7611   $172.76   $172.75   $172.80  
  7 purchase totals = $643.80       $690.1004   $690.10   $690.05   $690.10