If you go to the S&W website customer service section, you can download the owners manual for your "modern revolvers". The ammunition list on pages 9-11 lists any .38 Spl, .38 Spl +P or .357 Magnum loads as suitable for use.
+P+ loads are omitted because there are no industry standard pressure levels for them, so the factory will not warrant the suitability of the ammunition for use and will not warrant the weapon if used with that ammunition. While +P+ pressures probably shouldn't exceed .357 Magnum pressures, there's no guarantee of that since there's no standards. This is one of those cases where there's probably not a real world issue, but if there is, what you may spend on repairs will overwhelm possible ammo savings.
Having said that, .38 Spl +P+ ammunition was specifically developed for use in .357 Magnum revolvers to give Magnum performance without the ammunition having the dreaded "magnum" headstamp. Explaining the reasons for this would be lengthy and pointless at this time. However, due to the liability issues, the +P+ ammunition was only loaded under specific contract with the purchaser/user acknowledging that they assumed all liability for accelerated wear, damage, possibly injury or death (bad breath & body odor were probably included) resulting from the use of the ammunition.
From a practical standpoint, there is no real reason to bother using +P+ ammo when it would void your warranty even if your N frame will probably digest it nearly forever. There's plenty of mid range .357 Magnum ammo (Remington Golden Saber for example) that will probably match/exceed the performance of any .38 +P+ load.
