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 $133.98   3.565%   $138.7564   $138.76   $138.75   $138.80  
  Purchase #2 $26.52   7.635%   $28.5448   $28.54   $28.55   $28.50  
  Purchase #3 $248.56   13.698%   $282.6077   $282.61   $282.60   $282.60  
  Purchase #4 $154.35   12.265%   $173.281   $173.28   $173.30   $173.30  
  Purchase #5 $125.40   10.144%   $138.1206   $138.12   $138.10   $138.10  
  Purchase #6 $45.98   7.474%   $49.4165   $49.42   $49.40   $49.40  
  Purchase #7 $177.53   6.548%   $189.1547   $189.15   $189.15   $189.20  
  7 purchase totals = $912.32       $999.8817   $999.88   $999.85   $999.90