Rules of thumb are that ice can form in cumulus anytime. In summer likely to be light in winter can be severe. In stratus it's normally much less of an issue. In some cases high humidity without visible moisture on a cold day can cause ice (as mentioned above) again that's not usually too much of an issue.
I've had ice in Cu in May in South West Scotland at 5000 ft, I had ice last week over Cranfield climbing through Cu to FL 070.
The biggest risk by far is below rain cloud with an aircraft below zero. Freezing rain will really spoil your day.
BTW I'm an IMCR holder and would have been asking the same questions soon after my test, experience comes with time
