The issue that you describe where your prior merchant account provider billed you a downgrade rate for 90% of your transactions isn't really because they were "high-risk" (they weren't, you have no downgrades). It's an example of "marking up the downgrades" which is a strategy that unscrupulous merchant account providers use to increase their profit.
http://transfs.com/blog/marking-up-downgrades/What they really mean is that the interchange rate that your transaction qualified for is worse than the rate that they set to be your default or "qualified" rate. http://transfs.com/blog/what-is-interchange-again/
These billing tiers are a problem because there is no standard on what is "qualified" and what is "unqualified", it varies between processors and between customers within the same processor. http://transfs.com/blog/tiered-pricing-for-merchant-accounts...
There is a way to solve those billing tier ambiguities (which never are in the favor of the businessowner) - its called interchange plus pricing. Insist upon it, every processor can do it, for any size business.
Basically interchange is the wholesale rate that visa/mc charge the processor. In an interchange-plus pricing scheme the processor pass those charges on to you and explicitly disclose the markup you are paying above that wholesale rate. It's the best way to ensure no funny business is going on with your rates - http://transfs.com/blog/why-you-should-want-interchange-plus...
Only accept interchange plus or a no-downgrade flat percentage (like Paypal offers) - it will save you money and aggravation.