The pairs divide. AA, KK and QQ are high; JJ, TT, 99, 88 and 77 are middles; and 66, 55, 44, 33 and 22 are low pairs. 77/88 and 66 differ in pair brackets. Mid/low pairs often call for separate strategies, but that of 77/88 and 66 is similar.
66 limps: in feuds with others, outcomes are unclear. 66 win% is 56 - it wins 5600 of 10,000 hands. 66 loss% is 44 - it loses 4400 of 10,000 hands. There is 56% chance of winning and 44% chance of losing. Likewise, 77/88 win% is 70 - it wins 7,000 of 10,000 hands. 77/88 loss% is 30 - it loses 3,000 of 10,000 hands. There is 70% chance of winning and 30% chance of losing.
Limping is weakness, uncertainty and stealth. The danger is opponents using stealth to hide strength. Stealing with mid/low pairs risk running into stronger cards - leaving too much to chance in a skill game. Call pre-flop since raising such pairs escalates stack risks. Reach the flop for free when all check around, if possible, or cheap when bets are low.
Low pairs want to trip on the flop if it fails a steal pre-flop. If nothing trips, check, hoping for a free ride to the turn. If a player raises, or another re-raises, or if cards with higher win values than 66 present, then fold - since the potential to lose is seen in raises and re-raises of opponents and visible in the cards on the felt. It can be costly to chase missed pre-flop steals and missed trips on flop.
If a pair trips on the flop, the odds of winning climb. But, the next move is critical. Low trips are strong, but being low, are vulnerable to overhands like higher trips, straights, flushes and others. So, take a look at the board and analyze the flop well. If the potential to win is clear, then aggressively increase high betting in order to raise opponents out. Actively avoid going to the turn or river. Slow playing low trips can invite disaster for reasons above.
Position counts with mid/low pairs. It reveals players and folders; betters, raisers and re-raisers. It gives style data. Early limps can suffer losses to stacks. Later players have data availability, but this does not guarantee wins. Many fold 77/88 and 66 since the chance of losing is high. The problem with stats is they cannot tell you "when" a win or loss happens. So, if you play 77/88 or 66, be more careful than carefree. Playing hole cards with mid/low win percentages like 88/77 or 66 depends on strategy in the kind of game you play, e.g., cash game or tournament; or, e.g., your game position - early, mid or late; or your stack size, your opponents' stacks and pot size. Playing short stacks can put your game or tournament at immediate or imminent risk, as does playing against opponent deep stacks or large pots. This data and knowing how to use it is important to analysis and decision-making when playing pairs with low win percentages. So, despite their respective places in pair divides, similarities in playing the mid pairs 77/88 and low pair 66 should be clear.
Best Advice: Despite all possibilities - do not try to steal the blinds. Limp, but be prepared for quick fold at whatever stage...