If you are playing with a regular partner, you may introduce some conventions. This includes things like underbidding in front of partner and expecting them to adjust for it.
Many players I see always underbid and prefer to play a bagging game rather than risk getting set. The problem is that by underbidding, you are losing out on all those 10s that you would get if you bid your hand up. And they can make a difference in the end.
A good guide is to count 1 point for each ace and king, and any spade more than 3, and reduce a point if you have only one spade.
Nil bids should always be considered. If you have an ace or a king in a suit of 5 cards you almost certainly won't ever be forced to play it. With 4 you might but it's quite possible you won't. With 3 you'd never normally nil with the ace or king, but might risk it with the queen. With Qx in a suit you don't normally nil, Jx you might risk it.
I used to have a convention with partner that a 5 or 7 bid encourages nil opposite, so with a hand worth 5 but bad nil-cover you bid only 4. In my circles blind-nil was banned.
Back around 2003 I was rather obsessed with spades and was in communication with many of the more "expert" circles. There was one site that hosted "duplicate" spades with a regular weekly tournament. I won it many times.
In later stages we used to play "wiz", a variation where you had to bid your number of spades or nil. Used to play first to 500 which meant games were long and the luck of cards balanced out and it became far more of a skill game. In general I always hated the bagging game, and in wiz, the aim is always to set. bags only come into play if someone has nilled and then you generally concentrate more on covering it, trying to set it or setting the niller's partner rather than avoiding bags.