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 $19.99   6.437%   $21.2768   $21.28   $21.30   $21.30  
  Purchase #2 $14.56   4.729%   $15.2485   $15.25   $15.25   $15.20  
  Purchase #3 $97.49   10.651%   $107.8737   $107.87   $107.85   $107.90  
  Purchase #4 $242.88   6.468%   $258.5895   $258.59   $258.60   $258.60  
  Purchase #5 $29.76   11.241%   $33.1053   $33.11   $33.10   $33.10  
  Purchase #6 $123.89   4.519%   $129.4886   $129.49   $129.50   $129.50  
  Purchase #7 $163.43   12.199%   $183.3668   $183.37   $183.35   $183.40  
  7 purchase totals = $692.00       $748.9492   $748.96   $748.95   $749.00