"Compact" crank/cassette compatibility

When I select my next crank and cassette I would like it to be compact so I have more options at the easier-pedaling range. It's more important to me to have these easier gears, in particular for hills. Also, I simply don't use my large chainring very often and when I do, I'm rarely in the hardest cog.

Given the number of options (e.g. Ultegra SL - 2 crank options, 7 cassette options), I want to ensure I select a good working combination...i.e. would shifting be degraded (at all) if I selected a 50/34 and 12/27? Can I simply select any combination, or is there a rule of thumb I'm not aware of.

Thanks.

No rule of thumb. Any shimano compact crank and shimano cassette combo will work mechanically. Choose the combo/ratios that will work the best for you.

Depending on the kind of terrain you ride the 50/34 and 12/27 will work fine for hilly rides. I use a Shimano compact 50/34 with a SRAM 11/28 10 speed cassette. It works great on hills and flats