JBledsoe,

You can't use primer flattening as a warning sign for low pressure calibers such as the .35 Rem. By the time that pressure sign develops you may have already exceeded the strength limit of the thin brass case, primer cup, or the strength of weaker action types.
For bolt actions and high pressure calibers, flattened primers can work as a high pressure sign and if the primer pierces they safely handle the escaping gases much better than lever actions do.

Wear good shooting glasses every time you shoot if you plan to hot load the .35 Rem. in lever guns.