There is no "precedent" from 2008 because no new constitutional convention was made. The PM remains PM until resignation, regardless of his or her party's strength in the Commons. If the PM loses a confidence vote, e.g. on the Throne Speech, he can either resign or ask the GG for a new election. The latter would be extremely unusual and probably untenable immediately after an election, so resignation is more or less the only option.
In Ontario in 1985, it wasn't until two months after the election that the Liberals and NDP signed an agreement to defeat the Miller government. In the 1985 election, the PCs had won only a handful of more seats than the Liberals, but the Liberals had actually won more of the popular vote. There is absolutely no convention or precedent requiring any coalition or alliance need be made prior to an election result. Absolutely none.
In 2008, the Harper Conservatives won almost twice as many seats as the Liberals. I was a coalition supporter at the time, but it is certainly true that it was something of an ungainly arrangement. I think there is a lot of reasonable question about the constitutionality of prorogation at that time - certainly in the spirit of parliamentary supremacy anyway - but it's also true that the additional time resulted in the Liberals bailing and Ignatieff taking the leadership in something of an internal coup.
If this year the Conservatives do win a plurality of seats, there is a strong chance that the second-place finisher will be fairly close in the seat count. If so, I think it's fairly likely that the 1985 Ontario situation will occur for the first time at the federal level. If, however, there is a big spread between the Conservatives and the second-place party, it's going to be less certain. We might be more likely to see a brief Conservative minority, followed by a confidence defeat some time later, and then early elections.
To re-emphasize, no party leader can go to the GG to "request" the opportunity to form a government. The sitting PM remains PM until resignation, and afterwards it is at the GG's discretion who he requests to attempt to form a government.