There are a lot of creatures in Prophecy that get power/toughness bonuses if their owners lands are tapped.

1. Can a person tap out after declaring one of these creatures a blocker or attacker and boost the creature?
2. One specific card is the Keldon Berserker. It only gets the boost when attacking, can the boost occur any time during the combat phase. I.E. I declare it as an attacker with 2 lands untapped-- my opponent declares a blocker-- I then tap my two lands to play an instant-- at this point does my berserker get boosted?

Let's use Keldon Berserker and Spur Grappler as our guinea pigs:

KELDON BERSERKER
3R, Creature - Soldier, 2/3
Whenever Keldon Berserker attacks, if you control no untapped lands, it gets +3/+0 until end of turn.

SPUR GRAPPLER
2R, Creature - Beast, 2/1
Spur Grappler gets +2/+1 as long as you control no untapped lands.

What we've got here are triggered [404 and 410] and static abilities [405]. Triggered abilities tend to start with "when", "whenever", or "if". One of the things that help identify static abilities is the lack of a duration (like "until end of turn").

When a Keldon Berserker attacks, his trigger checks for untapped lands. If he finds no untapped lands, he gets trigger happy, and you put that ability on the stack. If the condition (no untapped lands) is still true, the Berserker gets his +3/+0 until end of turn no matter what happens to your lands.
The Spur Grappler gets and loses his bonus based on where your lands are at that time (continuously checking). If you're tapped out at any time, the Grappler gives himself the +2/+1 bonus.
You could even tap out after attacking or blocking for the Grappler if you want him larger. But in the case of the Berserker, you'll have to tap out before you declare attackers [308] in order to get the triggered bonus. In either case you can tap out during the Beginning of Attack step [307] and use that mana anytime until the end of attack [311] without mana burning.