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 $14.64   8.574%   $15.8952   $15.90   $15.90   $15.90  
  Purchase #2 $14.63   12.256%   $16.4231   $16.42   $16.40   $16.40  
  Purchase #3 $112.58   3.847%   $116.911   $116.91   $116.90   $116.90  
  Purchase #4 $17.95   8.388%   $19.4556   $19.46   $19.45   $19.50  
  Purchase #5 $198.98   11.150%   $221.1663   $221.17   $221.15   $221.20  
  Purchase #6 $106.29   12.490%   $119.5656   $119.57   $119.55   $119.60  
  Purchase #7 $6.73   9.128%   $7.3443   $7.34   $7.35   $7.30  
  7 purchase totals = $471.80       $516.7611   $516.77   $516.70   $516.80