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 $127.31   6.637%   $135.7596   $135.76   $135.75   $135.80  
  Purchase #2 $82.40   8.634%   $89.5144   $89.51   $89.50   $89.50  
  Purchase #3 $152.97   4.675%   $160.1213   $160.12   $160.10   $160.10  
  Purchase #4 $186.26   12.117%   $208.8291   $208.83   $208.85   $208.80  
  Purchase #5 $7.40   11.113%   $8.2224   $8.22   $8.20   $8.20  
  Purchase #6 $142.32   12.607%   $160.2623   $160.26   $160.25   $160.30  
  Purchase #7 $103.55   11.392%   $115.3464   $115.35   $115.35   $115.30  
  7 purchase totals = $802.21       $878.0555   $878.05   $878.00   $878.00