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 $63.86   3.642%   $66.1858   $66.19   $66.20   $66.20  
  Purchase #2 $196.57   4.807%   $206.0191   $206.02   $206.00   $206.00  
  Purchase #3 $67.65   7.649%   $72.8245   $72.82   $72.80   $72.80  
  Purchase #4 $131.56   12.148%   $147.5419   $147.54   $147.55   $147.50  
  Purchase #5 $63.37   3.648%   $65.6817   $65.68   $65.70   $65.70  
  Purchase #6 $12.63   10.845%   $13.9997   $14.00   $14.00   $14.00  
  Purchase #7 $21.83   10.124%   $24.0401   $24.04   $24.05   $24.00  
  7 purchase totals = $557.47       $596.2928   $596.29   $596.30   $596.20