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 $246.73   12.151%   $276.7102   $276.71   $276.70   $276.70  
  Purchase #2 $33.70   4.233%   $35.1265   $35.13   $35.15   $35.10  
  Purchase #3 $183.87   7.828%   $198.2633   $198.26   $198.25   $198.30  
  Purchase #4 $16.76   7.715%   $18.053   $18.05   $18.05   $18.10  
  Purchase #5 $59.91   3.214%   $61.8355   $61.84   $61.85   $61.80  
  Purchase #6 $24.98   7.706%   $26.905   $26.91   $26.90   $26.90  
  Purchase #7 $208.98   10.331%   $230.5697   $230.57   $230.55   $230.60  
  7 purchase totals = $774.93       $847.4632   $847.47   $847.45   $847.50