So stoked to see this thread revived. It was this very thread and the people I met through it which got me through the crucible that is Flight Planning.
is there a guideline to follow to figure out if the crosswind-caused headwind component is big enough to mention on the flightplan
I could not (and still cannot) help but correct for Effective TAS but I found that this always under two knots in Air Speed and three knots in GS (the difference between the two comes in rounding I've decided). Unless you are super retentive you shouldn't worry about it (likewise I found that calculating TAS I was always between 1 and 1.5 knots faster than the published answer as well as the Flight-Computer derived answer).
Long live AFPL; CASA's most positive contribution to the safety of air navigation.
FRQ CB