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 $234.99   9.412%   $257.1073   $257.11   $257.10   $257.10  
  Purchase #2 $172.37   11.777%   $192.67   $192.67   $192.65   $192.70  
  Purchase #3 $81.71   10.243%   $90.0796   $90.08   $90.10   $90.10  
  Purchase #4 $167.49   11.107%   $186.0931   $186.09   $186.10   $186.10  
  Purchase #5 $70.83   4.323%   $73.892   $73.89   $73.90   $73.90  
  Purchase #6 $66.67   13.792%   $75.8651   $75.87   $75.85   $75.90  
  Purchase #7 $174.87   6.763%   $186.6965   $186.70   $186.70   $186.70  
  7 purchase totals = $968.93       $1,062.4036   $1,062.41   $1,062.40   $1,062.50