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 $25.37   12.484%   $28.5372   $28.54   $28.55   $28.50  
  Purchase #2 $19.51   3.433%   $20.1798   $20.18   $20.20   $20.20  
  Purchase #3 $35.32   5.713%   $37.3378   $37.34   $37.35   $37.30  
  Purchase #4 $139.96   3.714%   $145.1581   $145.16   $145.15   $145.20  
  Purchase #5 $138.48   13.873%   $157.6913   $157.69   $157.70   $157.70  
  Purchase #6 $36.62   11.399%   $40.7943   $40.79   $40.80   $40.80  
  Purchase #7 $205.86   4.252%   $214.6132   $214.61   $214.60   $214.60  
  7 purchase totals = $601.12       $644.3117   $644.31   $644.35   $644.30