I'm glad you asked this before freaking out about it collapsing and using more resources than you need.
But people generally like to put supports because it irks people if the bridge seems to stand up without help
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A hunter who chases two rabbits will catch neither
Right now it looks kinda weird just floating up there so I will be making it look more realistic (I know, an odd thing to say about MC but you know what I mean). But it's a VERY long bridge so will take some time to get enough resources to pretty it up. (But I've been distracted by finding an abandoned mine (first time) and am going nuts gathering rails and trying to make a spider trap.)
The sooner the better though because I really don't like it as it is.
Thanks to everyone for your replies. I'm a newbie to MC (Christmas) and find these forums very helpful.