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 $19.74   6.373%   $20.998   $21.00   $21.00   $21.00  
  Purchase #2 $158.38   3.223%   $163.4846   $163.48   $163.50   $163.50  
  Purchase #3 $50.97   7.535%   $54.8106   $54.81   $54.80   $54.80  
  Purchase #4 $14.55   8.777%   $15.8271   $15.83   $15.85   $15.80  
  Purchase #5 $11.95   4.311%   $12.4652   $12.47   $12.45   $12.50  
  Purchase #6 $27.28   11.648%   $30.4576   $30.46   $30.45   $30.50  
  Purchase #7 $231.65   8.160%   $250.5526   $250.55   $250.55   $250.60  
  7 purchase totals = $514.52       $548.5957   $548.60   $548.60   $548.70