I think it's fine. I know I'm in a minority, but the way I look at it, if they don't pick it up at nursery they'll get it outside somewhere, whatever it is, colds, the various poxes - kids are forever touching stuff and putting their hands in their mouths, it's unavoidable. In terms of the one off spotty viral infections, we immunise against the dangerous ones, and the others (hand foot and mouth etc) you want to get as early as possible - I had a weird childhood and as a result didn't get some of them until I had kids myself, and they're really quite bad in adults where kids are ill for like 48 hours and then bounce back mostly.
With this in mind I will share with you a small piece of dodgy nursery sick kid cheating that I figured out. Lots of nurseries have a policy of not admitting kids who've had Calpol that morning, so you might have the situation where they wake up and they're like 37.8 or something and you want to give them Calpol as a precaution but then the nursery might not take them. But if you lie, and the kid gets worse, nursery's going to phone you at 10.30 and say "his temp's 38, can you pick him up and do you want us to give him Calpol" and you have to say no because it's not 4 hours since his last one so then they know you were telling porkies. So the best thing to do in that situation is to give them liquid ibuprofen instead. NOT if it's not indicated, and not if you suspect chickenpox, because ibuprofen and chickenpox is bad news - but if it's clearly a cold, yep, ibuprofen, and then if they want to give him calpol at 10.30 you can say OK.
Now I don my tin hat and wait for the flames to engulf me. I know ideally you don't do that, but sadly people's employers aren't always that understanding and in these times of precarious employment and redundancy and whatever, if you have to do it once or twice and it keeps you safer in your job, it's still in your kid's best interests.