As a general rule of thumb in the NBA a two point spread to have an edge, assuming that the lay each way is -110. You can probably shade that to a point and a half if the total money spread is 15 cents (example: -107 each way or -110 and -105). and also possibly on sides where the bottom number ranges from from -1.5 to -5 (example: -1.5 / +3). On totals, I'd want a two and a half point spread at -110, possibly two points on low totals or on a 15 cent money lay (example -110 -105).
Remember that these are just rules of thumb and my own opinion.