I've never been asked by station staff about a cycle reservation. I have occasionally been asked by train staff. In practice I've found it has made very little difference. I've been allowed on without one and I've been turned away with one.
Virgin are operating a close to no tolerance policy at Euston at the moment. Some of the staff are so ignorant that they'll only believe my Dahon folds when I actually fold it. Otherwise, no reservation, no entry.
I'm afraid some Virgin train staff are rather useless IMO.
At the outset to our Shetland tour we needed to catch the 06:45 from Rugby to Glasgow. Bikes booked weeks in advance, we arrive at the station for the 'train Manager' to tell us that he already has four bikes and we cannot board. This would have completely fucked our holiday as we had a connection in Glasgow then an overnight ferry from Aberdeen. There is all but no way we could have made that ferry. I held my ground, the station staff at Rugby had enough sense to contact Virgin Control, and wee Jimmy Crankie and his side kicks behaved like morons pursuing a line of argument such as, "Do you think you should be able to board the train just because you have reservations?". My answering "YES." half a dozen times to their repeated dim-wittery seened to confuse them even further.
Thankfully Virgin Control told wee Jimmy Crankie that he had to take us by either adding extra bikes, or by ejecting the unbooked bikes. He of course was the bottler type and eventually we were allowed onboard. I only delayed the train by about ten minutes.
I found the unbooked cyclists easily enough and had a good chat with them. They were a young asian couple off for their first C2C by the look and sounds of things. They were keen, committed and keen. They told me that they have never booked bikes on Virgin; In fact, they told me that they didn't even know that you needed to book bikes. I hope that they enjoyed their trip, and that they got home safely.
I have noticed previously at Euston that you call fairly easily roll up to and load your bike onto a Pendo without being challenged. I have done it with laden tourer and tandem.