This is a common concern of parents who sign. The real question, is would you prefer to teach a real living language to your baby, one they might continue with or one that is less rigid, comprised of some official signs and some made-up signs?
How strict you are depends on your goals with baby sign language and what you want from a signing program. Keep in mind that signs you make up on your own won’t be recognized by anyone but yourself and whomever you teach. If you teach ASL, then other people will understand your baby when they sign. Teaching made-up signs takes just as long as teaching an official sign, although with a little less flexibility. Some early sign programs used to make signs simpler for babies to do, but as more babies learned to sign over the years, it was discovered that babies could just as well make signs that adults did, only with a little less clarity. The signs that weren’t perfect at first where “approximated” and with time and consistent modeling by parents, sign clarity improved.
The vast majority of the signs included in our sign language program are easy for a baby to do and represent a balance between difficulty and the relative interest your baby will have in using the word. That is, words that your baby won’t really want to sign, have not been included and the words that remain, are the ones your baby can approximate with relative ease.
Just keep in mind that using an official sign language is a bit more difficult, but not so much so, that it’s worth making up signs specifically for babies. ASL and other sign languages offer many benefits that can’t be offered by made-up signs. Sign languages have specific rules and sentence structures, have many resources available and is common enough throughout your region that you are likely to come across other signers who enjoy the language as well.