You can use a "spent" case about 2 times, if you absolutely must dry fire your TC. After about 2x, use another spent case, then use another spent case, then another, etc. I'll bet you have a sack full of spent cases.
Snap caps are great for letting the hammers down on a double barrel shotgun that does not have exposed hammers to release the hammer spring. If you don't plan to store the double-barrel for several years, they are a waste of money.
There is some good scientific info out there that shows that compressed springs loose very little of there tension when stored for a while.
If I were worried about spring tension, I might buy some snap caps. Otherwise, they are only useful for some practice shooting in the living room, which could be done just as well with some "previously fired" cartridges.