Unless you were to make the unusual move of trying to fishless cycle a full scale heavily planted tank completely loaded with plants, the use of plants and decorations should have basically no effect on the actual cycling of the filter.
Here are some further comments:
Adding plants and decorations during fishless cycle:
Pros:
1) Makes the tank look nice.
(keeps you or other household members more entertained)
2) Plants might bring a tiny bit of beneficial bacteria.
3) You can experiment with different aquascaping arrangements.
Cons:
1) The decorations might get covered with algae and need to be scrubbed.
2) The plants might get covered with algae and need to be discarded.
3) The plants will make differences (usually minor) in the readings you are taking for the fishless cycle which might or might not confuse you.
Performing a blacked out bare tank fishless cycle:
Pros:
1) Greatest chance of getting no algae.
2) No skewing of measurement numbers during cycling.
Cons:
1) Big black ugly box to look at instead of a tank.
2) No chance of plants bringing in a few bacteria.
~~waterdrop~~