It does affect your credit score but nothing significant as filing for bankruptcy or not paying debt.
As long as you're financially responsible, such as paying it off on time and keeping the utilization less than 30% (anecdotal). You will be fine.
One of the reasons churning can affect your credit score is because the average age of the accounts goes down as you open more accounts (Your credit score will go back up as the revolving tradelines mature). I guess you can slow it down a little bit when you're trying to buy big-ticket items such as a house mortgage.
Hitting the minimum spend requirements for the sign-up bonus is a no-brainer for the money you're going to spend anyway.
For example, applying for
CIBC Aeroplan VI, you only need to hit $1,000 to earn 20k Aeroplan points + a buddy pass! A trip for two from YYZ-YVR costs 20.8k + $150 (Obviously, in economy). In cash it’ll probably cost around the $500-$600 mark so, not too shabby.
But if your net worth is north of $3.5m, it's probably not worth your time