I started doing sentences about half-way through heisig, (using
http://www.guidetojapanese.org/). There were a few kanji I couldn't write at that point, but I just learned to recognise them untill I learnt how to write them. Don't use ふりがな, you can still learn the readings of the kanji, even if you don't know how to write them. If you can, I would suggest you finish heisig as quick as you can, but if you motivation is started to suffer (like mine was) then starting to add sentences early might help.
If you have questions about AJATT or Heisigs method, at
This site pretty much everyone there uses AJATT so you might get better answers there
