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 $76.37   11.131%   $84.8707   $84.87   $84.85   $84.90  
  Purchase #2 $28.63   8.108%   $30.9513   $30.95   $30.95   $31.00  
  Purchase #3 $154.30   3.491%   $159.6866   $159.69   $159.70   $159.70  
  Purchase #4 $28.75   11.448%   $32.0413   $32.04   $32.05   $32.00  
  Purchase #5 $176.77   5.546%   $186.5737   $186.57   $186.55   $186.60  
  Purchase #6 $194.51   4.285%   $202.8448   $202.84   $202.85   $202.80  
  Purchase #7 $148.91   6.795%   $159.0284   $159.03   $159.05   $159.00  
  7 purchase totals = $808.24       $855.9968   $855.99   $856.00   $856.00