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 $16.35   3.174%   $16.8689   $16.87   $16.85   $16.90  
  Purchase #2 $63.29   4.538%   $66.1621   $66.16   $66.15   $66.20  
  Purchase #3 $174.79   6.508%   $186.1653   $186.17   $186.15   $186.20  
  Purchase #4 $92.27   8.790%   $100.3805   $100.38   $100.40   $100.40  
  Purchase #5 $90.96   13.557%   $103.2914   $103.29   $103.30   $103.30  
  Purchase #6 $145.27   12.724%   $163.7542   $163.75   $163.75   $163.80  
  Purchase #7 $26.60   12.336%   $29.8814   $29.88   $29.90   $29.90  
  7 purchase totals = $609.53       $666.5038   $666.50   $666.50   $666.70