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 $49.92   12.367%   $56.0936   $56.09   $56.10   $56.10  
  Purchase #2 $194.28   11.195%   $216.0296   $216.03   $216.05   $216.00  
  Purchase #3 $76.25   10.348%   $84.1404   $84.14   $84.15   $84.10  
  Purchase #4 $150.88   10.311%   $166.4372   $166.44   $166.45   $166.40  
  Purchase #5 $180.37   3.330%   $186.3763   $186.38   $186.40   $186.40  
  Purchase #6 $55.69   5.311%   $58.6477   $58.65   $58.65   $58.60  
  Purchase #7 $245.51   3.212%   $253.3958   $253.40   $253.40   $253.40  
  7 purchase totals = $952.90       $1,021.1206   $1,021.13   $1,021.20   $1,021.00