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 $19.99   3.499%   $20.6895   $20.69   $20.70   $20.70  
  Purchase #2 $18.85   13.434%   $21.3823   $21.38   $21.40   $21.40  
  Purchase #3 $164.74   8.305%   $178.4217   $178.42   $178.40   $178.40  
  Purchase #4 $149.36   8.166%   $161.5567   $161.56   $161.55   $161.60  
  Purchase #5 $234.85   11.787%   $262.5318   $262.53   $262.55   $262.50  
  Purchase #6 $114.33   7.491%   $122.8945   $122.89   $122.90   $122.90  
  Purchase #7 $184.37   9.381%   $201.6657   $201.67   $201.65   $201.70  
  7 purchase totals = $886.49       $969.1422   $969.14   $969.15   $969.20