One of my recent games was against an opponent who had blocked the side aisles with bombs so he could only move through the center. If there's one dumb setup for bombs then it's this one sine it would be very likely that his last two bombs would be in any corner. Anything else I could just hit with my strongest pieces. So I breached both walls and walked inside, taking everything while his strongest pieces were in the center aisle trying to guess the location of my spy.
Basically, it's not smart to put bombs in the front row since it's most likely costing your opponents his scouts to discover them...
But then where else? I've been thinking about that and have a few suggestions...
First of all, you should protect your flag with two or three bombs. This leaves three or four bombs to place randomly on your side. Avoid the use of patterns since your opponent will guess the pattern!
To deter lotto attacks, it's a good idea to put the bombs away from the front lines. Two or three on the last line and the others on the two center rows will put your opponents at risk if they try to walk through your side.
Do keep in mind that if you don't move any of the pieces in the two back lines then your opponent will not know which he can take and which are bombs. Since he most likely has spent all his scouts once those lines become more open, he will have to sacrifice his more expensive pieces to find those bombs. And if you have Lieutenants and Captains in those back rows, he will need to sacrifice pieces that are stronger. Or he'll be sending miners which almost any of your pieces will be able to hit.
Remember that you want to make bomb detection as expensive as possible for your opponent. This also means that you should avoid the columns for the center aisle as location for bombs, because he might still have a scout available that can check.
Sometimes, it also helps to fake a pattern. Put a sergeant in the back corners and put a bomb in front of them. Miners love to go hunting for flags in those locations. If they do, you take them and will still have enough bombs left to protect.
Really do avoid blocking one or more aisles with bombs since your opponent will, in general, just lose scouts on them and they're blocking your own attack routes! Even one bomb blocking an aisle will allow an opposing strong piece to post next to it, taking anything that passes through. If that piece is a Marshall, you will be forced to exchange Marshalls just to allow your other pieces to get through.
Some players like to put bombs on the front or second row, next to an aisle, hoping an attacking player will step sideways when they go through. It's reasonable effective and can win you a Captain or Major, if your opponent is a bit careless. Or even a Marshall if he's playing lotto. But that same bomb will block your routes between aisles because you have to step around it. And experienced players are less likely to attack those bombs with their stronger pieces. Then again, a bomb next to an aisle column in the second row can be very effective.
Random bomb patterns are annoying for your opponent since it slows down his attacks. There are ways to counter them, though. By exchanging pieces both sides will have a cleaner board and will have to start moving pieces from the back lines. When this happens, your opponent will have to look for a bomb in the third row. Once he finds one, the flag might be behind it, flanked by two more bombs. He could then send a miner to check one of those sides and if he's right, your flag is exposed.
This happens because your opponent will recognise this as a triangle pattern. Thus, with two or three bombs in the third row, your opponent will multiple, possible patterns and he might guess wrong.
In the past, I even used a setup where all bombs were next to one another, except one, starting from the left side. Four of those had a sergeant behind them and the fifth had a flag. The sixth bomb was used to close access through the side. Many strong pieces have fallen to those bombs!
But such an approach can't be used too often.