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 $236.75   13.482%   $268.6686   $268.67   $268.65   $268.70  
  Purchase #2 $102.83   8.243%   $111.3063   $111.31   $111.30   $111.30  
  Purchase #3 $73.37   10.815%   $81.305   $81.31   $81.30   $81.30  
  Purchase #4 $105.99   7.432%   $113.8672   $113.87   $113.85   $113.90  
  Purchase #5 $33.91   10.596%   $37.5031   $37.50   $37.50   $37.50  
  Purchase #6 $127.54   5.568%   $134.6414   $134.64   $134.65   $134.60  
  Purchase #7 $213.98   8.666%   $232.5235   $232.52   $232.50   $232.50  
  7 purchase totals = $894.37       $979.8151   $979.82   $979.75   $979.80