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 $222.45   12.364%   $249.9537   $249.95   $249.95   $250.00  
  Purchase #2 $20.65   6.107%   $21.9111   $21.91   $21.90   $21.90  
  Purchase #3 $165.71   7.699%   $178.468   $178.47   $178.45   $178.50  
  Purchase #4 $182.97   13.134%   $207.0013   $207.00   $207.00   $207.00  
  Purchase #5 $174.39   13.108%   $197.249   $197.25   $197.25   $197.20  
  Purchase #6 $57.67   3.471%   $59.6717   $59.67   $59.65   $59.70  
  Purchase #7 $46.27   7.292%   $49.644   $49.64   $49.65   $49.60  
  7 purchase totals = $870.11       $963.8988   $963.89   $963.85   $963.90