Terumah Gedolah should not be measured with precision but should be given באומד by estimation. One should not even use a vessel of known measurement to remove the Terumah Gedolah, unless one doesn't fill the vessel to a known line. (Terumot 1:7, Rambam Terumot 3:4)
Maaserot (Rishon, Sheni, Ani, and Min HaMaaser) should be given with high precision. The Mishna (Terumot 4:6) writes:
המונה משובח והמודד משובח ממנו והשוקל משובח משלשתן.
One who counts is [more] praiseworthy [than one who estimates]; one who measures is more praiseworthy than one who counts; one who weighs is more praiseworthy than all three of them.
The Shulchan Aruch (YD 331:24) takes this as referring to a item which can be evaluated in all three ways. If it is usually only evaluated in one of the ways, then that method is sufficient.
Rambam (Maaserot 1:14) summarizes nicely:
וכל המדקדק בשיעור משובח.
Whoever is exacting in measuring is praiseworthy.