Deca dick varies between each individual.
If you are that worried about it, then take clomid throughout the cycle @ 50mg's per day. Add HCG every few weeks as well (with nolvadex). This should help out a lot.
Also, order some Cialis or Viagra as a 'cautionary' measure for them 'special' moments. 2 grams of Arginine 30 minutes before you sex along with Cialis is awesome.
I think some bro's on here also take Maca throughout their cycle to help with the sex drive as well.
Keeep your deca cycle no longer than 10-12 weeks with the advice I listed above, and you should not have much problems. You will still need to have the normal post-cycle regimine lined up though...obviously.
BMJ