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 $77.93   8.202%   $84.3218   $84.32   $84.30   $84.30  
  Purchase #2 $32.80   4.773%   $34.3655   $34.37   $34.35   $34.40  
  Purchase #3 $17.55   6.533%   $18.6965   $18.70   $18.70   $18.70  
  Purchase #4 $80.78   6.400%   $85.9499   $85.95   $85.95   $85.90  
  Purchase #5 $42.35   12.758%   $47.753   $47.75   $47.75   $47.80  
  Purchase #6 $235.46   6.331%   $250.367   $250.37   $250.35   $250.40  
  Purchase #7 $147.52   6.561%   $157.1988   $157.20   $157.20   $157.20  
  7 purchase totals = $634.39       $678.6525   $678.66   $678.60   $678.70