You are, of course, free to cite which ever references that you think are more relevant. There is no requirement. The most important are listed at the bottom of this page:
http://theory.cm.utexas.edu/henkelman/research/saddle/
I'll highlight 4, but don't feel like you have to cite them all:
1. The original paper:
H. Jónsson, G. Mills, K. W. Jacobsen, Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions, in Classical and Quantum Dynamics in Condensed Phase Simulations, Ed. B. J. Berne, G. Ciccotti and D. F. Coker, 385 (World Scientific, 1998).
2. Improved tangent to avoid the use of a switching function to keep the path straight:
G. Henkelman and H. Jónsson, Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points, J. Chem. Phys., 113, 9978 (2000).
3. Climbing image to find precise saddles:
G. Henkelman, B.P. Uberuaga, and H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys., 113, 9901 (2000).
4. Efficient optimizers for the NEB and comparison to the string method:
D. Sheppard, R. Terrell, and G. Henkelman, Optimization methods for finding minimum energy paths, J. Chem. Phys. 128, 134106 (2008).