as you might know, people on netpd are using different operating systems, different distributions of pd and different ways to obtain their externals, libraries and abstractions. to ensure that people with other configurations can use your patches, you should consider a few things.

pd core

regarding the pd version, users have the choice of miller's/canonical pd, hans' pd-extended and the versions from the sourceforge cvs, either miller's main trunk, or the devel_0_39.
to put it straight, it does not matter which pd distribution you use, but the minimum version is 0.39, or a recent 0.38 version of pd-extended. so there's no reason not to include (list) in your patches for portability.

libraries used by netpd or netpd-patches

zexy and maxlib

first of all, we have netpd's dependencies, maxlib and zexy. those are needed for netpd and available for the usual pd operating systems (win, osx, linux), so you can use any of their objects in your netpd patches. maxlib will definitely stay a part of netpd for it's networking objects, the future of zexy is a bit uncertain, so always think twice about wether you can substitute a simple zexy object with builtin pd objects (for example abs~ is the same as (max 0)+(min 0)*-1).

iemmatrix

next, we'll have iemmatrix, it is needed for jamx, a sequencer used in many netpd-patches. using iemmatrix is fine with your patches then.
issues with iemmatrix:

[matrix] is an alias of [mtx]. the problem with it is, that on pd-extended you can only instanciate it after you created a [mtx] first. that's why using [mtx] is preferable.

iemlib

since a lot netpd-patches use iemlib now, it became an official netpd-lib somehow and it is also included in the pd-netpd-installers for osx and win.
issues with iemlib:

[iem_prepend_kernel] is not included in the version of iemlib, that comes actually with pd-extended, so don't use it (use [list prepend] instead). Don't use also all iemabs, that contain [iem_prepend_kernel].
[t3-del] is an alias of [t3_delay]. the problem with it is, that on pd-extended you can only instanciate it after you created a [t3_delay] first. that's why using [t3_delay] is preferable.

other libraries

of course can you use other externals than from the above in your netpd-patches, but be aware then, that other users might not have them installed. there are some netpd-users, who just use netpd and aren't pd cracks and also don't know how to install an external. that's why it makes sense in my eyes (roman) to use only 'standard'-libs. but this is not a rule, of course. feel free to experiment with externals. if many people like your patche(s), and maybe other netpd-users use the same external in their patches, chances are big, that this library will become a 'standard'-lib too. there is no official policy about this topic yet, since still only a few people are using netpd and decisions are made based on discussions.
please also check, that the library you are using is available on all platforms. it is known that some externals do work only on one platform (mostly on linux).