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 $6.33   7.734%   $6.8196   $6.82   $6.80   $6.80  
  Purchase #2 $78.54   9.685%   $86.1466   $86.15   $86.15   $86.10  
  Purchase #3 $144.78   10.469%   $159.937   $159.94   $159.95   $159.90  
  Purchase #4 $72.75   7.724%   $78.3692   $78.37   $78.35   $78.40  
  Purchase #5 $71.31   5.192%   $75.0124   $75.01   $75.00   $75.00  
  Purchase #6 $44.30   11.391%   $49.3462   $49.35   $49.35   $49.30  
  Purchase #7 $113.27   7.785%   $122.0881   $122.09   $122.10   $122.10  
  7 purchase totals = $531.28       $577.7191   $577.73   $577.70   $577.60