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 $27.79   13.812%   $31.6284   $31.63   $31.65   $31.60  
  Purchase #2 $207.97   8.326%   $225.2856   $225.29   $225.30   $225.30  
  Purchase #3 $176.70   8.782%   $192.2178   $192.22   $192.20   $192.20  
  Purchase #4 $56.86   11.826%   $63.5843   $63.58   $63.60   $63.60  
  Purchase #5 $157.90   6.432%   $168.0561   $168.06   $168.05   $168.10  
  Purchase #6 $62.50   3.360%   $64.6   $64.60   $64.60   $64.60  
  Purchase #7 $42.81   4.200%   $44.608   $44.61   $44.60   $44.60  
  7 purchase totals = $732.53       $789.9802   $789.99   $790.00   $790.00