A lot depends on the loading situation and the quality standard of the weld. As mentioned in another answer the yield stress of the filler metal is often higher than that of the base metal.
However this needs to be qualified by the fact that the welding process can, in some circumstances, modify the material properties of the base metal in the heat affected zone (especially for cold worked or high alloy steels).
There is also the question of weld defects such as incomplete penetration, cold starts, inclusions, porosity etc etc as well as the fact that some alloys require specific pre and/or post weld treatment.
In very general terms weld defects are often related to crack propagation problems and so are more of a concern with structures where fatigue and other cyclic or impact related modes of failure are a concern.
Critical welding applications are well covered by application/industry specific standards which cove the whole process as well as testing and inspection procedures.
For good quality welds in low alloy steels with good penetration and no major defects it is generally reasonable to consider a welded joint as if it was a continuous piece of metal unless you're working to a code which states otherwise.
Having said that I recently quoted for a maintenance job for aluminium alloy generator cages where welds are a known failure point.
However this is all highly contextual which is why we have specific welding codes which define procedures and testing protocols and specific situational rather than having a single fudge factor for 'welds'.