If ATC are happy with a backtrack, it should be included in the line up / take-off clearance.
SOP where I work is the big boys taxy to the end for departure, light aircraft go to an intermediate point which still gives them 1500m+ or 2000m+ depending on the runway in use. Means you can get the light stuff away in front of the big guys thereby negating the 3 minute vortex wake separation sometimes. If a C172 or similar decided they wanted to backtrack from that point, with or without asking, he would probably incur some form of displeasure and probably get delayed!