I'd say its always worth it to mention to your leaders that you're interested in cross training. However you need to think long and hard about if you really want to going Flex. When I left, we had a hard time retaining Flex TMs because none of them realize how hard and stressful it is and how you have to be able to operate independently. A lot of people seem to think it's just walking around the store "shopping" all day and you get to hide from leadership. It's quite the opposite. You're not working as part of a team really, it's up to you to meet your goals and get everything done. It's up to you to reach out right away if you need help. Flex TMs who wait until they're getting in trouble to bring up the fact that they needed help or they didn't know how to do something, don't last long. I used to tell my team all the time to use their walkies and call for TMs/TLs in that area first for help finding something, instead of waiting until the LOD checks in when you're approaching goal time. Now the decision has to be made to try to make the sale and miss the goal time or INF the item and make goal time.
You will absolutely get blamed if you can't meet goal time and fulfillment metrics. No one wants to hear how you were having problem in the backroom, the RFID wasn't working, you kept getting stopped by guests, etc. If you're not prepared for that, don't cross train. My advice is to make nice with the ETL or TL in charge of flex. No matter how much other ETLs yell at you, it's the Flex ETL you ultimately have to impress. I constantly had other ETLs and even the STL complaining about one of my Flex TMs. At one point my STL even hinted that maybe it was time to performance her out. I had to remind them multiple times (and come with reports/metrics/schedules backing me up) that she was the only Flex TM for most of her shift and she works full time. Yes her gross INF number was higher than other Flex TMs but that's because she does more batches. I had to bring out the INF percentages to show that her's was the lowest. No one cares about Flex (including the STL) until shit starts hitting the fan.