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 $9.99   4.538%   $10.4433   $10.44   $10.45   $10.40  
  Purchase #2 $168.88   9.428%   $184.802   $184.80   $184.80   $184.80  
  Purchase #3 $162.99   12.330%   $183.0867   $183.09   $183.10   $183.10  
  Purchase #4 $130.69   10.800%   $144.8045   $144.80   $144.80   $144.80  
  Purchase #5 $60.38   6.817%   $64.4961   $64.50   $64.50   $64.50  
  Purchase #6 $72.77   6.652%   $77.6107   $77.61   $77.60   $77.60  
  Purchase #7 $212.38   4.469%   $221.8713   $221.87   $221.85   $221.90  
  7 purchase totals = $818.08       $887.1146   $887.11   $887.10   $887.10