This procedure, called Claiming or Conceding as the case may be, is common in Contract Bridge (both Rubber and Tournament). However it is also error prone for beginners and can be used unethically by those morally challenged. In consequence there are several Laws governing the process to protect both sides.
When playing casual Bridge all players typically want to play as many hands as possible, so claims and concessions are frequent. I imagine your Spades group feels likewise. However if your group is unfamiliar with making Claims and concessions, however, I would suggest laying out a few ground rules in advance to avoid misunderstandings and hurt feelings. The following excerpts from the Laws of Rubber Bridge may assist with that. You may need to make minor adjustments to compensate for the absence of a DUmmy in Spades.
Law 68 – Declarer’s Claim or Concession of Tricks
Declarer should not make a claim or concession if there is
any doubt as to the number of tricks to be won or lost.
Law 69 – Procedure Following Declarer’s Claim or Concession
... declarer must ... make a comprehensive statement as to his proposed plan of play, including the order in which he will play the remaining cards.
Declarer’s claim or concession is allowed, and the deal is scored accordingly if both defenders agree to it. ....
When his claim or concession is not allowed, declarer must play on, leaving his hand face up on the table. ....
The objective of subsequent play is to achieve a result as equitable as possible to both sides, but any doubtful point must be resolved in favor of the defenders.
Note that the above is from the Laws of Rubber Bridge - the non-tournament variety of the game. Stricter constraints apply to Duplicate (Tournament) Bridge, where a Director is available to both adjudicate as well as ensure that fairness is maintained for the players at other tables.