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 $17.35   3.698%   $17.9916   $17.99   $18.00   $18.00  
  Purchase #2 $103.68   3.368%   $107.1719   $107.17   $107.15   $107.20  
  Purchase #3 $82.88   3.592%   $85.857   $85.86   $85.85   $85.90  
  Purchase #4 $100.50   4.460%   $104.9823   $104.98   $105.00   $105.00  
  Purchase #5 $186.78   3.703%   $193.6965   $193.70   $193.70   $193.70  
  Purchase #6 $150.59   5.360%   $158.6616   $158.66   $158.65   $158.70  
  Purchase #7 $45.67   13.830%   $51.9862   $51.99   $52.00   $52.00  
  7 purchase totals = $687.45       $720.3471   $720.35   $720.35   $720.50