I don't do modern, so I can't speak to that. For vintage, I find that as long as the same guide is used for both ends of a trade, it should work out. If the trade is based on Beckett hi or Beckett lo or PSA Ex5 on both sides of the trade, it should end up being fair. Where it's unequal is when separate guides are used to judge value.
One thing I have noticed is that BV of '70s cards is consistently lower than what you can get at online auction. I'll be chasing a $20 card of a hof player in NM and it ends up going for 50, 60 or sometimes even three figures. And there are some common cards whose special interest the guides just don't take into account - like the Claude Raymond fly down, or Glenn Burke rc, or the '75 mvps, to name a few.