I.e. if you bought something that cost $5 you could just hand the $5 to the cashier and they could just avoid ringing you up and pocket the money. In contrast if the object cost $4.99 or $4.95 say they would have to ring it up in the till so they could open it to provide change to the customer.
I heard this on Tom Scott's Podcast "Lateral" recently.
The article says you can. It's one of the "supported conventions".
This looks silly, and many people, including me, have taught themselves to recognize this pattern and round the price correctly without a mental effort.
Some people, of course, fall for it; I suppose younger kids are heavily affected.
Apologies.
Just wanted to really get across that most importantly, the l and f are definitely both silent.
Reminds me of one of my favourite coins I had in my possession as a child, a "thrup'ny bit"!
For the uninitiated this was a pre-decimalisation three pence coin.