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 $20.84   4.375%   $21.7518   $21.75   $21.75   $21.80  
  Purchase #2 $121.46   5.340%   $127.946   $127.95   $127.95   $127.90  
  Purchase #3 $64.93   10.618%   $71.8243   $71.82   $71.80   $71.80  
  Purchase #4 $16.49   8.631%   $17.9133   $17.91   $17.90   $17.90  
  Purchase #5 $211.56   13.166%   $239.414   $239.41   $239.40   $239.40  
  Purchase #6 $22.80   6.605%   $24.3059   $24.31   $24.30   $24.30  
  Purchase #7 $76.54   5.448%   $80.7099   $80.71   $80.70   $80.70  
  7 purchase totals = $534.62       $583.8652   $583.86   $583.80   $583.80