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 $240.85   13.652%   $273.7308   $273.73   $273.75   $273.70  
  Purchase #2 $116.59   12.489%   $131.1509   $131.15   $131.15   $131.20  
  Purchase #3 $94.89   8.707%   $103.1521   $103.15   $103.15   $103.20  
  Purchase #4 $38.72   12.408%   $43.5244   $43.52   $43.50   $43.50  
  Purchase #5 $248.90   8.474%   $269.9918   $269.99   $270.00   $270.00  
  Purchase #6 $23.73   3.585%   $24.5807   $24.58   $24.60   $24.60  
  Purchase #7 $44.70   7.373%   $47.9957   $48.00   $48.00   $48.00  
  7 purchase totals = $808.38       $894.1264   $894.12   $894.15   $894.20