Are you placing more weight on patient risk factors such as age >65 or co-morbidities?
In what is obviously an answer without data, my approach would be to consider the use of wbc growth factor based upon the risk of the therapy being administered and its clinical setting (as we have historically done) without attempting to factor in the "risk" of another viral illness, as we have not historically changed our approach in patients at high risk of influenza etc. If therapy is not being administered with curative intent/improvement in OS, delay or dose modification is frequently a more reasonable approach.
There are many functions of G-CSF, including repression of T-cell and NK cell function. Unless you are certain that growth factors are not modifying the immune network to the detriment of viral clearance—there is no data that growth factors help clear viral infections.