Hello @Wendy112,
That’s a great question! The answer is going to depend on your personal casting ethics. This is because ethics (aka what you believe is “right” or “wrong” to do in magick) can vary depending on the individual and the path they follow.
For example, someone who follows a Wiccan path may follow the Wiccan Rede: “An harm ye none, do what ye will”. This is often interpreted to mean that, so long as your magick does not intend to harm, you are free to cast however you please, with or without permission from the recipient.
That being said, some witches believe it is important to ask permission before casting on someone, even if it a positive or helpful spell. Other witches may choose to seek permission from deities or divine forces and leave the spell results up to Their decision. Yet other witches may believe it’s fine to cast on others without needing permission.
At the end of the day, it’s up to you (and any rules of a tradition if you follow one) to set your personal magickal ethics and decide what feels right for you 
I’ve heard this belief before - I think it’s following the same idea as not sharing wishes when someone blows out cake candles on their birthday, lest the wish not come true. Some covens and traditions may follow this practice, but it’s certainly not the only way!
Here in the forum, you’ll find many people who are very open with their spellwork. We share spells, recipes, ritual experiences, and more in the name of passing along wisdom and helping one another. In my years of experience, I don’t think sharing or not sharing has any effect on whether the spell works or not. Plenty of the spells I’ve shared pre-spellwork, mid-process, and post-spellwork worked just fine. In fact, sometimes the tips or advice I got from others helped me cast better and improved the experience 
I’d encourage you to try different things and see what works best for you and your beliefs, Wendy! 
Happy casting and blessed be 