I think this is something that will vary from couple to couple. In my experience, most ALL tops want information about how something feels to their bottom. They would want to know if something "hurts like a brick" so they can make a fully informed decision about whether or not to use it. And it will vary from couple to couple whether the bottom has veto power.
In a more D/s or M/s type relationship, the bottom is not likely to have much, if any, veto power. The bottom's choice is to consent to discipline/authority, or to leave (or stop doing ttwd). They don't always have ANY choice about what shape ttwd takes. But they CONSENT to being in a relationship with those power dynamics.
I think many ppl here on the board are more on the domestic discipline side of the spectrum and less the M/s side of the spectrum. In those forms of ttwd, I think a bottom has much more say over implements and punishments in general. Also, the fact that a lot of us bottoms "convinced" our tops to do ttwd, and have tops who knew nothing about it before we introduced them to it, has an affect (I think) on the shape of the power dynamic.
For me, it's a bit of a cross between the two. In reality, S cares a great deal what I think, and I could probably veto pretty much any object. In my opinion, though, that's because S CHOOSES to care what I have to say. Based on what I've consented to, and the power exchange between us, she isn't under (in my opinion, though not so much hers, I think) any OBLIGATION to consider my opinion. My choice is to revoke my consent to ttwd and call it off.
Also, there's other elements that come into play. Yes, a spanking is supposed to hurt, and be miserable. There are some implements that hurt in a...I don't know how to say it, like a wrong way, that makes me feel like I CAN'T take it. S could listen to that and stop spanking me with it--or she might use it just for real serious offenses. And sometimes, even though we're just supposed to take it, sometimes humanity gets in the way. Sometimes, once these tops get rolling on their own, they'll sometimes spank us when we don't think we deserve it, or more/harder than we think we've really earned. And then suddenly, all that great stuff about submission goes right out the window, and we just want to try to make that stop, or at least not happen again!