Them durned pharyngeal fricatives

Designing my lang, I am thinking about keeping voiced and unvoiced
pharyngeal fricatives (/X\/ & /?\/) in the mature language (they occur
in the protolang anyway), but I'm also trying to evaluate their effect
on the phonemic system. As far as I know, and comparing with recordings
of Haa' and :ayn in Arabic, I'm pronouning them correctly, but I have
the problem (or feature) than my tongue is very tense while saying them,
and held in a back position, so that combinations like /X\A/ and /X\O/
are much easier than say /X\i/. Yet at the same time, I can't see why my
tongue need be like this since the sounds are made by a constriction of
the pharynx - unless that's done partly with the tongue.

So the question is: is the tongue tensing during these sounds a
necessary feature of them, or is it an artifact of however I taught (or
brutalised, connived or cajoled) my vocal tract to make them?

I suspect the former, since Ha and `ayin in Ancient Hebrew were
reportedly these sounds, and there are some special rules for these
letters - e.g. a schwa (in AH not necessarily a /@/) following these
letters has to be 'about half a real vowel' in length, and there
are some cases where these letters prefer that the quality of this
half-schwa is that of the vowel /a/ or /A/ I think.

So, am I right or am I right? ;)

Seriously, writing this letter I feel like I've convinced myself, but
maybe someone else can reassure or disabuse me of the notion?

Stephen