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 $83.79   11.127%   $93.1133   $93.11   $93.10   $93.10  
  Purchase #2 $248.94   11.229%   $276.8935   $276.89   $276.90   $276.90  
  Purchase #3 $59.97   6.122%   $63.6414   $63.64   $63.65   $63.60  
  Purchase #4 $192.89   12.703%   $217.3928   $217.39   $217.40   $217.40  
  Purchase #5 $215.83   5.565%   $227.8409   $227.84   $227.85   $227.80  
  Purchase #6 $36.95   8.466%   $40.0782   $40.08   $40.10   $40.10  
  Purchase #7 $58.36   11.845%   $65.2727   $65.27   $65.25   $65.30  
  7 purchase totals = $896.73       $984.2328   $984.22   $984.25   $984.20