I just played a series and the best combo method I found without any bonuses is methodical and slow tapping to clear screens. Large screens seem not to be worth it, as a full screen combos at 8,000 maybe at 18x multiplier, but 6 cubes value at 600-900 at the same range. small screens that are quick clears are more desirable once you have a large multiplyer I think. Easier to track each tap as well with more white space if they are small groupings. I played at the max zoom for this usually just because I didn't want to risk losing the combo, and it because it did seem to have good returns. Further zooms might be better once you have apick.
watch out for the FB or tool icon to pop up as it will kill your combo timer.
*edit - apparently on the twitter account they suggest the best way to get rid of the FB icon is to log into facebook.... clever.
I got 200,000 coins in about 5-10 minutes by going carefully into a semi-cleared section and screen clearing and not missing a combo. my total tap meter was at about 17,000 and during that stretch I got 200,000 coins. I imagine that if I could buy the chisel at 300,000 that my screen clear time would go much faster and thus my multiplier would add up faster on the clear combo. I'd guess the 25 square tool would super bump the screen clear.
I think the best way to go is to get the pick tools and clear quickly while maximizing combos so that you can afford the tool the next time + coin gain.