Filling out the appropriate W-8 is standard process for investing in a US partnership and this should be provided upon admittance as a Limited Partner. The reason being, this form captures many details which could become relevant depending on the activity of the partnership and the reporting obligations that arise as determined by the IRS. 

There are withholding rates determined based on the data provided should a withholding obligation become necessary. It is not best practice to collect this only when the obligation arises as it could delay the timeliness of filing obligations including K-1 delivery which are time sensitive.