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 $18.40   13.114%   $20.813   $20.81   $20.80   $20.80  
  Purchase #2 $52.95   11.167%   $58.8629   $58.86   $58.85   $58.90  
  Purchase #3 $160.56   9.714%   $176.1568   $176.16   $176.15   $176.20  
  Purchase #4 $150.89   7.581%   $162.329   $162.33   $162.35   $162.30  
  Purchase #5 $26.69   12.710%   $30.0823   $30.08   $30.10   $30.10  
  Purchase #6 $46.93   11.535%   $52.3434   $52.34   $52.35   $52.30  
  Purchase #7 $141.42   4.273%   $147.4629   $147.46   $147.45   $147.50  
  7 purchase totals = $597.84       $648.0503   $648.04   $648.05   $648.10