Best way to store parameters that changes over the course of time

Consider the following scenario: We have a function (let's call it service_cost) that performs some sort of computations.

In that computations we also use a variable (say current_fee) witch has a certain value at a given time (we get the value of that variable from an auxiliary table - fee_table). Now current_fee could stay the same for 4 months, then it changes and obtains a new value, and so on and so forth. Of course I would like to know the current fee, but also should be able to find out the fee that was 'active' days, months, years before...

So, one way of organizing the the fee_table is

create table fee_table as (
 id number,
 valid_from date,
 valid_to date,
 fee number
)

And then at any given time - if I want to get the current fee I would:

    select fee into current_fee form 
    fee_table where trunc(sysdate) between valid_from and valid_to;  

What I don't like about the solution above, is that it is easy to create inconsistent entries into fee_table - like:

-overlapping time periods (valid_from-valid_to) e.g. (1/1/2012 - 1/2/2012) and (15/1/2012-5/2012)

-no entry for current period

-holes in between the periods e.g. ([1/1/2012-1/2/2012],[1/4/2012-1/5/2012])

etc.

Could anyone suggest a better way to handle such a scenario? Or may be - if we stick with the above scenario - some kind of constraints, check, triggers etc upon the table to avoid the inconsistencies described?

Thanks.