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 $46.63   12.764%   $52.5819   $52.58   $52.60   $52.60  
  Purchase #2 $232.87   13.691%   $264.7522   $264.75   $264.75   $264.80  
  Purchase #3 $182.99   12.869%   $206.539   $206.54   $206.55   $206.50  
  Purchase #4 $60.38   6.193%   $64.1193   $64.12   $64.10   $64.10  
  Purchase #5 $72.83   8.540%   $79.0497   $79.05   $79.05   $79.00  
  Purchase #6 $150.91   4.159%   $157.1863   $157.19   $157.20   $157.20  
  Purchase #7 $140.51   10.814%   $155.7048   $155.70   $155.70   $155.70  
  7 purchase totals = $887.12       $979.9332   $979.93   $979.95   $979.90