i wouldn't go and stick my private key into any and all applications. some are more trusted than others. if you use Android, i would recommend using an application called Amber. it's a private key manager. it holds your key for you so that you don't have to give it to applications. sadly nothing like this exists on iOS. if you use a laptop or desktop, you can use a lot of nostr apps on the web and use a web browser extension to hold your private key and sign into apps that way. it's more secure than entering your key. check out https://nostrapps.com to see what else is offered. i use an extension called Nostr Connect.