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 $30.66   13.499%   $34.7988   $34.80   $34.80   $34.80  
  Purchase #2 $30.47   8.242%   $32.9813   $32.98   $33.00   $33.00  
  Purchase #3 $6.71   12.229%   $7.5306   $7.53   $7.55   $7.50  
  Purchase #4 $138.55   6.267%   $147.2329   $147.23   $147.25   $147.20  
  Purchase #5 $30.25   11.177%   $33.631   $33.63   $33.65   $33.60  
  Purchase #6 $153.42   3.568%   $158.894   $158.89   $158.90   $158.90  
  Purchase #7 $157.49   4.217%   $164.1314   $164.13   $164.15   $164.10  
  7 purchase totals = $547.55       $579.2000   $579.19   $579.30   $579.10