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 $249.98   10.546%   $276.3429   $276.34   $276.35   $276.30  
  Purchase #2 $225.71   6.260%   $239.8394   $239.84   $239.85   $239.80  
  Purchase #3 $143.39   9.446%   $156.9346   $156.93   $156.95   $156.90  
  Purchase #4 $193.34   4.421%   $201.8876   $201.89   $201.90   $201.90  
  Purchase #5 $75.38   7.438%   $80.9868   $80.99   $81.00   $81.00  
  Purchase #6 $179.32   3.234%   $185.1192   $185.12   $185.10   $185.10  
  Purchase #7 $223.94   13.576%   $254.3421   $254.34   $254.35   $254.30  
  7 purchase totals = $1,291.06       $1,395.4526   $1,395.45   $1,395.50   $1,395.30