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 $77.78   3.108%   $80.1974   $80.20   $80.20   $80.20  
  Purchase #2 $112.76   10.402%   $124.4893   $124.49   $124.50   $124.50  
  Purchase #3 $177.84   7.553%   $191.2723   $191.27   $191.25   $191.30  
  Purchase #4 $64.96   7.324%   $69.7177   $69.72   $69.70   $69.70  
  Purchase #5 $45.38   5.362%   $47.8133   $47.81   $47.80   $47.80  
  Purchase #6 $51.99   9.574%   $56.9675   $56.97   $56.95   $57.00  
  Purchase #7 $250.35   7.443%   $268.9836   $268.98   $269.00   $269.00  
  7 purchase totals = $781.06       $839.4411   $839.44   $839.40   $839.50