In short, yes. Hundreds of thousands of Soviet soldiers were stationed across the Eastern Bloc, largely as a means to enforce Communism in the states. And on the occasions that trouble did start, they were definitely utilised, as the mentioned 1956 and 1968 uprisings showed. It also can be seen when one considers that by the end of the 1980s Gorbachev said he would no longer use the Red Army to prop up Communist regimes, and when this was tested and proven true in 1989, the whole Eastern Bloc collapsed in increasingly quick succession.

While Soviet military power certainly was important in maintaining the Communist states in Eastern Europe, I'd like to step in and challenge the assumption in your question that the Soviet Union really 'controlled' any of these countries.

It is overly simplistic to view the post-war history of Eastern Europe as a 40-year period of Soviet domination. Certainly, at the time that they were established (late 40s/early 50s), these Communist states were supported and assisted by the Soviet Union, and developed along Stalinist lines (economic plans which focused on heavy industry, the establishment of an effectively one-party state, political show trials, etc). Soviet influence can be seen and felt in the establishment of these states; I'm not denying this. But once the Communist states had established themselves, the leaderships and political climate of each state began to take on a distinctive flavour of their own - sure, there were general similarities between them, but each regime was distinct in its own way. We can begin to see this in the responses of the different regimes to de-Stalinisation from 1956 - whilst the Soviet Union pushed for a degree of economic and political liberalisation, some states (most notably Czechoslovakia) were highly resistant to this. Again, coming back to Czechoslovakia, pressure for military intervention in 1968 also came from other Warsaw Pact leaders and high-ranking members of the Czechoslovak Communist Party: Brezhnev/the Soviets were not the only ones who wanted to stop the tide of liberalisation initiated by the Prague Spring.

During its brief existence, the Eastern Bloc certainly lay in the shadow of the USSR. Once the Soviets announced that they would no longer intervene in the internal affairs of the European Communist states, those regimes collapsed fairly quickly because they were no longer viewed as legitimate by their citizens and could no longer call upon the Soviet Union to back them up. Fundamentally, the regimes relied on Soviet control to stay in place, yes, but this is different to saying that the Soviets controlled these states through military power. Narratives of Soviet domination provide convenient readings of post-war European history, but ultimately this type of narrative obscures rather than clarifies what the situation in Eastern Europe actually was.