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 $128.81   7.163%   $138.0367   $138.04   $138.05   $138.00  
  Purchase #2 $68.86   5.176%   $72.4242   $72.42   $72.40   $72.40  
  Purchase #3 $40.87   13.729%   $46.481   $46.48   $46.50   $46.50  
  Purchase #4 $204.57   11.610%   $228.3206   $228.32   $228.30   $228.30  
  Purchase #5 $49.47   9.296%   $54.0687   $54.07   $54.05   $54.10  
  Purchase #6 $71.71   8.626%   $77.8957   $77.90   $77.90   $77.90  
  Purchase #7 $19.35   13.829%   $22.0259   $22.03   $22.05   $22.00  
  7 purchase totals = $583.64       $639.2528   $639.26   $639.25   $639.20