I'm in the situation where I get 90% of my salary for the first 6 weeks, then peanuts the basic maternity rate after that.
These numbers aren't exact or true, but say my salary is £23k and that's reduced to £20k to take into consideration the fact I get childcare vouchers.
Means that during my 6 weeks of 90% maternity pay I'll only get the pro rata of £18k rather than £20.7k like I would have done if I'd stopped my childcare vouchers and not had any salary sacrifice, which would raise my average earnings. So over that 6 week period, I lose just under £200.
However, if I stop the childcare vouchers I won't then be able to restart them before maternity leave. Not messing with them more than once a year seems to be a pretty standard rule for the companies I've worked at. Over 9 months maternity (what I'm planning to take) I'd get approximately £2.25k of vouchers.
Given the choice between just under £200 in my salary during those 6 weeks or £2.25k of childcare vouchers over 9 months of maternity leave, I know which one I'd rather have. 
However if you have an employer who is going to pay 90% for six months, or you earn a huge salary, or you're only taking a very short maternity leave, then it may well be worthwhile to stop the vouchers.
I'm just saying that you need to make sure you check out your own circumstances and the fine print in your contract. Don't just cancel any childcare vouchers without doing all the maths very carefully. :)