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 $248.72   11.289%   $276.798   $276.80   $276.80   $276.80  
  Purchase #2 $83.93   12.395%   $94.3331   $94.33   $94.35   $94.30  
  Purchase #3 $108.51   11.668%   $121.1709   $121.17   $121.15   $121.20  
  Purchase #4 $171.81   7.223%   $184.2198   $184.22   $184.20   $184.20  
  Purchase #5 $133.73   8.599%   $145.2294   $145.23   $145.25   $145.20  
  Purchase #6 $46.49   7.611%   $50.0284   $50.03   $50.05   $50.00  
  Purchase #7 $111.29   11.338%   $123.9081   $123.91   $123.90   $123.90  
  7 purchase totals = $904.48       $995.6877   $995.69   $995.70   $995.60