Reference AWS 2008 D1.1 Figure 6.11 "Hole type IQI or Wire IQI on source side may be placed anywhere along the joint.
Reference ASME Section V:
(d) IQI Placement for Welds -- Wire IQIs. The IQI(s)
shall be placed on the weld so that the length of the wires
is perpendicular to the length of the weld. The identification
numbers and, when used, the lead letter "F," shall not be
in the area of interest, except when geometric configuration
makes it impractical.

Reference B31.3 2006
344.5 Radiographic Examination
344.5.1 Method. Radiography of castings is covered
in para. 302.3.3. Radiography of welds and of components
other than castings shall be performed in
accordance with BPV Code, Section V, Article 2.

Dunno where you read that, but it's not required in any code I am aware of.

(both references are from radiation source)

I wonder if your information is referring to source side verses film side placement?

~thirdeye~

qaqcmpm

I dont know any US codes which require this - maybe some old British codes or standards do such as BS3971.  Unfortunately I dont have acccess to it now.

IQI's were placed at the end of the diagnostic film length (readable portion of the film) and with the thinnest wire nearest the end of the 'graph. That way the very worst case film sensitivity (thinnest visible wire) was indicated. Doesnt seem like anybody bothers about this anymore.