From my understanding there are some differences you'll have to adapt to depending on which client you use. For example, I am pretty sure there are differences in how gumps in particular are drawn between Classic and Enhanced. While they'll still work and function the same way, they will look different or have content misaligned.
I could be wrong on this as I do not use EC, but I vaguely recall reading it somewhere and there is code in some gump files that differentiate between the user being on CC or EC which I assume it is meant to draw them properly depending on client. Maybe it was there to take advantage of EC only gump art? I am not sure. Either way, you can change/update any gump to determine the client and adjust appropriately.