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 $49.80   13.622%   $56.5838   $56.58   $56.60   $56.60  
  Purchase #2 $178.43   7.509%   $191.8283   $191.83   $191.85   $191.80  
  Purchase #3 $126.31   9.597%   $138.432   $138.43   $138.45   $138.40  
  Purchase #4 $146.82   5.148%   $154.3783   $154.38   $154.40   $154.40  
  Purchase #5 $23.64   6.486%   $25.1733   $25.17   $25.15   $25.20  
  Purchase #6 $173.55   5.419%   $182.9547   $182.95   $182.95   $183.00  
  Purchase #7 $90.57   5.695%   $95.728   $95.73   $95.75   $95.70  
  7 purchase totals = $789.12       $845.0784   $845.07   $845.15   $845.10