It's your choice if you think that you can still win your group games without playing your best players, I like to get the job done and get through, You can always use your subs too take off your best players if your winning so easily.
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for better training and chance to win ... i play reserves only when the titular player is injured or sanctioned :) so ... play only with main team !
Your out the cup? Then i would try to play my fast trainers in both cl and league, and use green rests so they play. I always mix my squad but keep the best trainers on as many games as possible. Obviously itll depend how competitive your league, or whatever comp u feel is more important.