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 $126.59   11.244%   $140.8238   $140.82   $140.80   $140.80  
  Purchase #2 $46.51   12.445%   $52.2982   $52.30   $52.30   $52.30  
  Purchase #3 $15.77   4.370%   $16.4591   $16.46   $16.45   $16.50  
  Purchase #4 $11.49   7.553%   $12.3578   $12.36   $12.35   $12.40  
  Purchase #5 $82.36   10.605%   $91.0943   $91.09   $91.10   $91.10  
  Purchase #6 $94.68   7.191%   $101.4884   $101.49   $101.50   $101.50  
  Purchase #7 $221.41   3.842%   $229.9166   $229.92   $229.90   $229.90  
  7 purchase totals = $598.81       $644.4382   $644.44   $644.40   $644.50