The current recommendation is that you stop penicillin prophylaxis after the age of 25 years. After this age any infection should be treated as if you never had rheumatic fever. Taking Penicillin as and when you have an infection is not appropriate unless your doctor feels that it is the appropriate drug. Most colds and coughs in adults are due to viruses and therefore an antibiotic including penicillin is not helpful.
Antibiotics should be taken (and that too the appropriate antibiotic) only if you have a bacterial infection. All antibiotics should be taken under medical supervision to ensure that they are being prescribed appropriately so as to reduce the likelihood of bacteria developing resistance to the antibiotic.