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 $231.31   12.378%   $259.9416   $259.94   $259.95   $259.90  
  Purchase #2 $242.45   8.868%   $263.9505   $263.95   $263.95   $264.00  
  Purchase #3 $96.49   8.268%   $104.4678   $104.47   $104.45   $104.50  
  Purchase #4 $77.85   7.448%   $83.6483   $83.65   $83.65   $83.60  
  Purchase #5 $7.80   3.167%   $8.047   $8.05   $8.05   $8.00  
  Purchase #6 $84.80   7.749%   $91.3712   $91.37   $91.35   $91.40  
  Purchase #7 $53.64   5.338%   $56.5033   $56.50   $56.50   $56.50  
  7 purchase totals = $794.34       $867.9297   $867.93   $867.90   $867.90