Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $232.86   12.298%   $261.4971   $261.50   $261.50   $261.50  
  Purchase #2 $53.97   12.155%   $60.5301   $60.53   $60.55   $60.50  
  Purchase #3 $212.40   3.358%   $219.5324   $219.53   $219.55   $219.50  
  Purchase #4 $195.78   12.848%   $220.9338   $220.93   $220.95   $220.90  
  Purchase #5 $72.34   13.220%   $81.9033   $81.90   $81.90   $81.90  
  Purchase #6 $216.94   11.356%   $241.5757   $241.58   $241.60   $241.60  
  Purchase #7 $158.58   7.380%   $170.2832   $170.28   $170.30   $170.30  
  7 purchase totals = $1,142.87       $1,256.2556   $1,256.25   $1,256.35   $1,256.20