I think in this kind of retail culture, you don't get paid to sign people up, you get literally nothing, it should be okay to stop pushing after the initial no.
If this were a real sales environment and cashiers had to have some sales experience and you get a small commission, I'd certainly be more persuasive, not so much aggressive, but persuasive. Cashiers at Target aren't anywhere near close to sales people and I think it's kind of stupid to put that kind of pressure on them to sell to get nothing in return, especially if it's a 17 year old just trying to get some part time work in.
Since I want to get involved in a sales experience I certainly try, to see if I can better myself as a salesman and as a speaker, but that's the only reason why I push past the first no, usually a 2nd no will shut me up and I'll continue with small-talk.
I think it's perfectly fine to ask "Are you saving 5% with our redcard?" and get told yes or no and leave it at that or go from there. My GSTLs ask that but they push it because they're graded on how many RCs they get in a shift, so it certainly matters to them, but I think it's inappropriate to continue to pressure TMs into getting 1 redcard every 100 transactions and pressure should definitely ease up on everyone.
While it's certainly doable to some people at Spot, I find a lot more people just don't want a redcard in my area and will never sign up for one even if it meant they get to live an extra long life. I also find a lot of TMs are burnt out from not being salespersons trying to sell a debit/credit card and get told no almost 90% of the time, they just stop caring because it doesn't mean anything anymore.