It's a help alright but I changed springs before without it. I just kept the spring compressed and tightened the nut with a "snipe nosed" pliers as hard as I could, might not be the right thing to do but it seems to have worked
.
When I said about filing a nut to leave two teeth on it I didn't think it through properly, it wouldn't really work.
Instead you could make a tool like Dave said (using a socket) or get a bit of metal tubing and file it down to form the two teeth and then weld a nut to the top of it. You could then use a spanner to tighten the nut on the strut rod and still be able to hold it in the centre with an allen key.

When I said about filing a nut to leave two teeth on it I didn't think it through properly, it wouldn't really work.
Instead you could make a tool like Dave said (using a socket) or get a bit of metal tubing and file it down to form the two teeth and then weld a nut to the top of it. You could then use a spanner to tighten the nut on the strut rod and still be able to hold it in the centre with an allen key.