First of all I'd probably say "why bother?" It is the top part of your penis that has the most nerve endings - the ones that give you pleasure. The "deep throat" thing is just a fetish - not that there is anything wrong with it.
Many women find it difficult to achieve. Personally, I'm happy with 1/2 way or maybe more with her using her hand on the bottom part of the shaft, fondling my testicles, or below.
Again, though, you don't have the nerve endings at the base that give you the same pleasure. It's great she wants to do this but why not just enjoy the blow job (which many men don't get to do) and say you can do without the full insertion?