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 $246.53   11.562%   $275.0338   $275.03   $275.05   $275.00  
  Purchase #2 $11.39   9.847%   $12.5116   $12.51   $12.50   $12.50  
  Purchase #3 $190.91   3.743%   $198.0558   $198.06   $198.05   $198.10  
  Purchase #4 $58.41   3.366%   $60.3761   $60.38   $60.40   $60.40  
  Purchase #5 $101.64   13.760%   $115.6257   $115.63   $115.65   $115.60  
  Purchase #6 $203.25   12.747%   $229.1583   $229.16   $229.15   $229.20  
  Purchase #7 $117.44   11.128%   $130.5087   $130.51   $130.50   $130.50  
  7 purchase totals = $929.57       $1,021.2700   $1,021.28   $1,021.30   $1,021.30