With, say, 40 seats, they'd have to, if Labour have enough of course.
Otherwise, why on earth are they standing for Westminster? If they hold the balance of power and refuse to form an functioning administration forcing another election they'd look like prize pri cks, with no reason for people to vote for them again.
You're right though, being in coalition could damage them in the long run as they won't be able to just stand and point any more. It did for the LDs and could do for the SNP too.