Assuming I have plenty of Greens to do so then yes. I've not been able to farm many this season and have used the ones I have had to put towards the rest of my team to make them competitive in the league, but I hope for easier league draws in the future.
If I can get around 1000 Greens a season from farming once Foster hits 28 year old then I intend to keep Power Training him. Hopefully he will also continue to pick up MOM awards, as each one he does pick up gives him an extra cone so its one less than the 75 cones needed to get that next star up.
He's only 19 now, so a long time until I have to consider Foster slowing down, but I'd like to keep him 9 star (and high 9 star) until the day he retires unless it becomes too costly. I am converting Tokens to Greens every time I have more than 10,000 Tokens, so as long as I still have 10K in Tokens I will continue to invest in Foster.