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 $246.83   12.754%   $278.3107   $278.31   $278.30   $278.30  
  Purchase #2 $98.25   6.377%   $104.5154   $104.52   $104.50   $104.50  
  Purchase #3 $193.65   4.571%   $202.5017   $202.50   $202.50   $202.50  
  Purchase #4 $164.33   8.680%   $178.5938   $178.59   $178.60   $178.60  
  Purchase #5 $243.52   9.517%   $266.6958   $266.70   $266.70   $266.70  
  Purchase #6 $31.81   9.727%   $34.9042   $34.90   $34.90   $34.90  
  Purchase #7 $49.27   9.454%   $53.928   $53.93   $53.95   $53.90  
  7 purchase totals = $1,027.66       $1,119.4496   $1,119.45   $1,119.45   $1,119.40