Theres a lot of established heuristics on this subject, and a few points I can toss in to help you construct an optimal mana base for Standard, Modern, Pioneer, or whatever MTG format you prefer
However, certain cards had abilities that let you cast them as instants before this, beginning with [c]King Cheetah[/c], in Visions
If you'd like to dive into a bit of tactical planning, check out our guide to the best MTG Commanders - or nab a few free digital cards with our regularly updated list of MTG Arena codes.
So no, you cant just cheese the rules and wait until you roll an 8 to pay the mana
It also brings up lots of hilarious flavor implications with regards to what actually constitutes a crime - for example, stabbing and murder are obviously crimes, but so too are giving someone a wedding ring, having a secret romantic rendezvous, and (for extra irony points) making an arrest