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 $249.68   12.146%   $280.0061   $280.01   $280.00   $280.00  
  Purchase #2 $47.35   3.702%   $49.1029   $49.10   $49.10   $49.10  
  Purchase #3 $115.32   9.716%   $126.5245   $126.52   $126.50   $126.50  
  Purchase #4 $155.93   6.735%   $166.4319   $166.43   $166.45   $166.40  
  Purchase #5 $177.96   5.731%   $188.1589   $188.16   $188.15   $188.20  
  Purchase #6 $58.99   10.140%   $64.9716   $64.97   $64.95   $65.00  
  Purchase #7 $189.83   4.575%   $198.5147   $198.51   $198.50   $198.50  
  7 purchase totals = $995.06       $1,073.7106   $1,073.70   $1,073.65   $1,073.70