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 $13.87   10.146%   $15.2773   $15.28   $15.30   $15.30  
  Purchase #2 $125.85   8.746%   $136.8568   $136.86   $136.85   $136.90  
  Purchase #3 $124.42   9.176%   $135.8368   $135.84   $135.85   $135.80  
  Purchase #4 $63.40   11.341%   $70.5902   $70.59   $70.60   $70.60  
  Purchase #5 $90.78   13.229%   $102.7893   $102.79   $102.80   $102.80  
  Purchase #6 $76.81   6.534%   $81.8288   $81.83   $81.85   $81.80  
  Purchase #7 $90.66   4.873%   $95.0779   $95.08   $95.10   $95.10  
  7 purchase totals = $585.79       $638.2571   $638.27   $638.35   $638.30