put very bluntly it depends what the points structure of your particular league is.
if you play in one like the TVL, where everything is weighted toward batting first, then you insert the opposition only on a sub 100 pitch.
if you play in a straight 50 or 40 over job...I think a good bet is to ask your senior seamer. Seriously - they'll either give you a clear sign that they're up for it in which case bowl, or they'll be noncommital/hedge their bets. Then, bearing in mind the value of scoreboard pressure at lower levels of the game, bat...