As per Lattice Technical Note TN1144, VCCIB and VCCOB are only used to supply input/output termination voltage to match the transmission lines.
By default, if a channel is unused during the IPexpress PCS generation phase, the autoconfig file (.txt) will power down this channel. As a result, whether you terminate VCCIB/OB of the unused channels or leave them floating makes no difference.
Another situation is where you enable a channel in IPexpress and terminate VCCIB/VCCOB to power. If you later decide not to use the channel in hardware, then terminating VCCIB/VCCOB also has a minimal effect due to the lack of activity on that channel.