I would definitly put in any AG liner on any sunny day warmer than 12 celcius (probably 60 fahrerheit, but my conversion may be off.) I would definitly not worry about the temp of the fill water. The warmth of the sun and the shop vac will definitly take care of any stretching you need. Note: I did say sunny day. When the sun is out liners seem to be much stretchier (trust me, thats a word). If you are putting it in on an overcast day I would look for a bit warmer one.

Brad
www.waterworkspools.com