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 $28.97   6.621%   $30.8881   $30.89   $30.90   $30.90  
  Purchase #2 $21.49   11.735%   $24.0119   $24.01   $24.00   $24.00  
  Purchase #3 $123.50   12.725%   $139.2154   $139.22   $139.20   $139.20  
  Purchase #4 $160.40   4.581%   $167.7479   $167.75   $167.75   $167.70  
  Purchase #5 $180.27   3.475%   $186.5344   $186.53   $186.55   $186.50  
  Purchase #6 $28.90   13.579%   $32.8243   $32.82   $32.80   $32.80  
  Purchase #7 $29.75   5.105%   $31.2687   $31.27   $31.25   $31.30  
  7 purchase totals = $573.28       $612.4907   $612.49   $612.45   $612.40