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 $9.99   9.776%   $10.9666   $10.97   $10.95   $11.00  
  Purchase #2 $148.63   13.411%   $168.5628   $168.56   $168.55   $168.60  
  Purchase #3 $97.54   4.704%   $102.1283   $102.13   $102.15   $102.10  
  Purchase #4 $148.53   9.624%   $162.8245   $162.82   $162.80   $162.80  
  Purchase #5 $21.30   4.526%   $22.264   $22.26   $22.25   $22.30  
  Purchase #6 $94.65   11.350%   $105.3928   $105.39   $105.40   $105.40  
  Purchase #7 $28.72   9.534%   $31.4582   $31.46   $31.45   $31.50  
  7 purchase totals = $549.36       $603.5972   $603.59   $603.55   $603.70