A lot depends on the Marriott chain you are staying in. In general, at non-FS hotels like RIs and CYs, I'm never upgraded by default. (Note that RI upgrades actually aren't a guaranteed elite benefit anyway.) But at CYs I've found that asking when I check-in about a suite upgrade, will quite often get me one if they have one available. (My last two stays in Overland Park was at a CY there and after asking, I was upgraded to a King Suite both times.) For RIs, I call MR to make my reservations and ask the agent if they will call the hotel to see if they will give me a complimentary upgrade. That has worked fairly well for me as well.

At FS hotels, if it is more of a business hotel, about the best you are going to do is a CL room, which given you already have CL access isn't much. But note that most FS Business oriented Mariotts don't tend to have much to upgrade persons to anyway. But if I'm staying at a Resort and would like to try for an upgrade, I will call MR to get the GM's email address. I'll send the GM a nice email letting her/him know that I'll be coming (usually with my family) and while not guaranteed, would appreciate any upgrades or other extra benefits they can offer. Quite often I will get some type of upgrade out of that (i.e. ocean front, corner, balcony, etc.)

If you don't ask? You might still get some type of upgrade. But I have found that taking a proactive stance can get me a much better upgrade (quite often beyond what a normal elite upgrade should be.) ^