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 $129.32   7.348%   $138.8224   $138.82   $138.80   $138.80  
  Purchase #2 $83.34   8.606%   $90.5122   $90.51   $90.50   $90.50  
  Purchase #3 $93.81   5.630%   $99.0915   $99.09   $99.10   $99.10  
  Purchase #4 $196.87   11.853%   $220.205   $220.21   $220.20   $220.20  
  Purchase #5 $74.91   3.484%   $77.5199   $77.52   $77.50   $77.50  
  Purchase #6 $49.61   6.145%   $52.6585   $52.66   $52.65   $52.70  
  Purchase #7 $184.64   13.353%   $209.295   $209.30   $209.30   $209.30  
  7 purchase totals = $812.50       $888.1045   $888.11   $888.05   $888.10