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 $222.68   12.266%   $249.9939   $249.99   $250.00   $250.00  
  Purchase #2 $182.33   10.230%   $200.9824   $200.98   $201.00   $201.00  
  Purchase #3 $167.94   6.644%   $179.0979   $179.10   $179.10   $179.10  
  Purchase #4 $203.35   12.711%   $229.1978   $229.20   $229.20   $229.20  
  Purchase #5 $159.34   5.597%   $168.2583   $168.26   $168.25   $168.30  
  Purchase #6 $188.67   9.713%   $206.9955   $207.00   $207.00   $207.00  
  Purchase #7 $62.81   5.827%   $66.4699   $66.47   $66.45   $66.50  
  7 purchase totals = $1,187.12       $1,300.9957   $1,301.00   $1,301.00   $1,301.10