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 $11.53   11.472%   $12.8527   $12.85   $12.85   $12.90  
  Purchase #2 $241.25   9.192%   $263.4257   $263.43   $263.45   $263.40  
  Purchase #3 $225.87   7.335%   $242.4376   $242.44   $242.45   $242.40  
  Purchase #4 $63.69   9.708%   $69.873   $69.87   $69.85   $69.90  
  Purchase #5 $167.61   9.109%   $182.8776   $182.88   $182.90   $182.90  
  Purchase #6 $156.76   13.696%   $178.2298   $178.23   $178.25   $178.20  
  Purchase #7 $18.42   11.661%   $20.568   $20.57   $20.55   $20.60  
  7 purchase totals = $885.13       $970.2644   $970.27   $970.30   $970.30