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 $248.44   11.406%   $276.7771   $276.78   $276.80   $276.80  
  Purchase #2 $35.78   7.349%   $38.4095   $38.41   $38.40   $38.40  
  Purchase #3 $239.57   8.565%   $260.0892   $260.09   $260.10   $260.10  
  Purchase #4 $118.54   4.492%   $123.8648   $123.86   $123.85   $123.90  
  Purchase #5 $117.32   11.671%   $131.0124   $131.01   $131.00   $131.00  
  Purchase #6 $153.70   12.363%   $172.7019   $172.70   $172.70   $172.70  
  Purchase #7 $197.74   8.800%   $215.1411   $215.14   $215.15   $215.10  
  7 purchase totals = $1,111.09       $1,217.9960   $1,217.99   $1,218.00   $1,218.00