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 $20.32   7.124%   $21.7676   $21.77   $21.75   $21.80  
  Purchase #2 $18.38   7.434%   $19.7464   $19.75   $19.75   $19.70  
  Purchase #3 $238.71   4.264%   $248.8886   $248.89   $248.90   $248.90  
  Purchase #4 $139.89   5.484%   $147.5616   $147.56   $147.55   $147.60  
  Purchase #5 $140.66   3.737%   $145.9165   $145.92   $145.90   $145.90  
  Purchase #6 $171.33   9.107%   $186.933   $186.93   $186.95   $186.90  
  Purchase #7 $28.46   12.134%   $31.9133   $31.91   $31.90   $31.90  
  7 purchase totals = $757.75       $802.7270   $802.73   $802.70   $802.70