As I see it, regardless of what date a new rMBP is announced there are going to be some lags, at least until I hand over the credit card. To wit:
I'd fall over in a dead faint if the new laptop were available on announcement date. Give it a month. Then there is at least a few to several weeks waiting for the early adopters to weigh in on their new purchase. I want to hear what people have to say before I shell out $3k on a new machine that's essentially a backup for a nMP.
If there is significant new technology - eGPU - it will take even longer for the third party market to catch up. Maybe months. Of course, on that front I'd probably have to buy some new monitors to take advantage. Not in the budget this year.
Other new tech such as USB-C/TB3 might be easier to suss out. I suppose there's a chance that I won't like what Apple has wrought. It's happened before.
If there's a March announcement I'd consider buying in June.
I'm in the luxurious position of not really needing a new machine - my 2012 8,1 runs just fine with 16GB RAM, a 160GB SSD boot drive and a terabyte HDD in place of the superdrive.
Biggest need for me is more memory to run VMs. 24GB would be ideal.
So I'll just wait it out. Curious, but patient.