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 $211.56   13.353%   $239.8096   $239.81   $239.80   $239.80  
  Purchase #2 $108.60   5.842%   $114.9444   $114.94   $114.95   $114.90  
  Purchase #3 $80.46   9.433%   $88.0498   $88.05   $88.05   $88.00  
  Purchase #4 $205.64   12.598%   $231.5465   $231.55   $231.55   $231.50  
  Purchase #5 $127.56   13.475%   $144.7487   $144.75   $144.75   $144.70  
  Purchase #6 $165.48   10.278%   $182.488   $182.49   $182.50   $182.50  
  Purchase #7 $18.55   9.748%   $20.3583   $20.36   $20.35   $20.40  
  7 purchase totals = $917.85       $1,021.9453   $1,021.95   $1,021.95   $1,021.80