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 $67.70   12.213%   $75.9682   $75.97   $75.95   $76.00  
  Purchase #2 $140.67   8.230%   $152.2471   $152.25   $152.25   $152.20  
  Purchase #3 $58.39   13.410%   $66.2201   $66.22   $66.20   $66.20  
  Purchase #4 $66.69   11.401%   $74.2933   $74.29   $74.30   $74.30  
  Purchase #5 $240.41   12.328%   $270.0477   $270.05   $270.05   $270.00  
  Purchase #6 $165.36   12.364%   $185.8051   $185.81   $185.80   $185.80  
  Purchase #7 $139.81   12.342%   $157.0654   $157.07   $157.05   $157.10  
  7 purchase totals = $879.03       $981.6469   $981.66   $981.60   $981.60