The system recommended by those using the cutting machines is to hold the selvedges together and move right and lift until the fold is even. This appears to work in most cases. However, if pattern is important in cutting your pieces then the fabric should be lined up with the pattern even if it is slightly off grain. When I cut strips on the cutting machine I always tare the fabric, line the edges up with selvedge and use the steam iron to gradually work the grain in line. As indicated above, grain is not necessarily that important in cutting some patchwork shapes. Off grain is usually made when the fabric is refolded in manufacturer's folding machines and is especially caused when long lengths of fabric are stitched together. If that cut is not reasonably straight then all the fabric folded after that point is crooked. I have seen this at the manufacturers. I think it is better to grit your teeth (so to speak) and tare the fabric to make sure it is on grain.