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 $120.93   13.706%   $137.5047   $137.50   $137.50   $137.50  
  Purchase #2 $55.34   3.268%   $57.1485   $57.15   $57.15   $57.10  
  Purchase #3 $156.37   7.591%   $168.24   $168.24   $168.25   $168.20  
  Purchase #4 $85.79   10.270%   $94.6006   $94.60   $94.60   $94.60  
  Purchase #5 $192.81   4.629%   $201.7352   $201.74   $201.75   $201.70  
  Purchase #6 $226.50   5.512%   $238.9847   $238.98   $239.00   $239.00  
  Purchase #7 $12.67   11.457%   $14.1216   $14.12   $14.10   $14.10  
  7 purchase totals = $850.41       $912.3353   $912.33   $912.35   $912.20