If you're only topping where the reshes are bad, not the good grass, I wouldn't worry. If you're topping a whole field of grass, I'd probably let it wilt a day or two before putting sheep on.
Just be aware that reshes are fantastic shelter for sheep, especially with young lambs, so don't top them all out of existence unless there is other good shelter! If they're huge and strong, but you do still want shelter, you could top 2/3 this year, leaving patches for the sheep to use over winter and in spring. Then next year, top the bits you left this year, plus go over half of what you did this year again. Something like that.