Slopp, your enthusiasm is singularly endearing.
True, the problem with an all peasant garrison is it affords you no immediate first-response to a full-fledged uprising, but you'll see that coming from miles away (and it usually will only happen through neglect on your part). There will always be cities which rebel with regularity, but you keep down the squalor, put in a good governor, and don't build up the walls (no stone if you can), then you'll be in business.
The only cities that should really upgrade defenses and have real armies in them are your frontier regions; those in danger of being beseiged. But as your expansion increases these will naturally stop being "frontier" or "backwater" outpost settlements and become inner provinces, allowing you to relieve them of their "frontline" status of importance. They will no longer be on the vanguard of your empire.
Also, if you think you need an army around, keep training crap loads of great units and/or buying mercs so you can deal with those pesky rebels who inevitably pop up and be close by in case a province gets really upset and gives your people the civil boot. It'll rarely get that bad, but rebels are very frequent late in the game as organized opposition becomes scarce, so you'll want patrolling armies. Besides, by then you'll be swimming in cash so blowing tons of money on newly-trained troops won't bat an eyelash.
Of course, by the time you get to that point, your empire will probably have dominated 2/3 of the map.
Have fun wiping Rome's filth from the face of terra firma!
"All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us?"
"Sometimes, a view from sinless eyes,
Centers our perspective and pacifies our cries..."