I don't believe it would significantly change the stress in the beam unless it's a REALLY big lag bolt or you're hanging a LOT of weight from it. How big of a lag bolt and what sort of weight are you asking about? It's probably only designed for a 10psf live load on the bottom chord.
In a truss, the main "truss" stress is usually tensile throughout the entire lower member (and can be over 4500lbs of tension for an SPF 2x4!), and splitting with a pre-drilled lag bolt is really no worse than having a naturally ocurring knot in the wood, which is taken into account. It would depend on the type of truss, the design stresses, and a lot of other factors to say for certain. It's probably OK as the trusses are expected to be perforated with screws and nails and designed with this in mind, but, if in doubt, ask the engineer who stamped the truss drawing!
FYI, don't ever center-bore or otherwise cut/drill a truss member anywhere without permission from the engineer. For a joist or beam, center-boring through the precise center is the best place structurally to do it, as stress increases with the cube of distance- boring a hole twice as far from the center weakens the beam 10x as much! For TJIs, honestly, it doesn't matter, so long as it's in the web.
[This message has been edited by SteveFehr (edited 01-04-2007).]
