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 $118.70   8.211%   $128.4465   $128.45   $128.45   $128.40  
  Purchase #2 $90.67   5.335%   $95.5072   $95.51   $95.50   $95.50  
  Purchase #3 $124.51   10.248%   $137.2698   $137.27   $137.25   $137.30  
  Purchase #4 $27.49   10.492%   $30.3743   $30.37   $30.35   $30.40  
  Purchase #5 $84.69   11.666%   $94.5699   $94.57   $94.55   $94.60  
  Purchase #6 $128.38   12.675%   $144.6522   $144.65   $144.65   $144.70  
  Purchase #7 $7.57   9.272%   $8.2719   $8.27   $8.25   $8.30  
  7 purchase totals = $582.01       $639.0918   $639.09   $639.00   $639.20