No worries on designating that the funds contributed are for Roth 2015 or 2016. When you open/contribute to a Roth, the account is opened as a Roth and you are required to designate whether the funds are for 2015 or 2016.
One thing I would caution you about...Form 8606. We have been contributing through a backdoor Roth since 2010. Our first accountant didn't think we needed to file the form, even though I kept asking. An old dog not learning new tricks, I suppose. It continued to concern me so in 2013, I switched to an accountant that supposedly knew how to correctly complete this form. Last July, IRS sent us a tax notice for the non-deductible contributions. Had to pay the accountant another $350 to straighten that out. Asked him to file corrected Form 8606's for all past years ($50 each year for each tax payer) to be certain that everything is documented but he recommended that we wait until our 2016 tax return to do that because he was afraid that all the repeated "issues" would target us for audit. :-o
It seems that many accountants weren't knowledgeable when the backdoor Roth first came about. So please be sure you or your accountant properly file this form, because it might come back to haunt you, in a big way, years from now when the IRS tries to tax you again. :-o
If I had it to do over, I probably wouldn't have bothered. But get that Form 8606 correctly done and you should be good!