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 $61.98   7.766%   $66.7934   $66.79   $66.80   $66.80  
  Purchase #2 $161.42   13.529%   $183.2585   $183.26   $183.25   $183.30  
  Purchase #3 $230.93   11.832%   $258.2536   $258.25   $258.25   $258.30  
  Purchase #4 $131.61   4.689%   $137.7812   $137.78   $137.80   $137.80  
  Purchase #5 $201.76   13.633%   $229.2659   $229.27   $229.25   $229.30  
  Purchase #6 $165.27   9.534%   $181.0268   $181.03   $181.05   $181.00  
  Purchase #7 $33.75   3.440%   $34.911   $34.91   $34.90   $34.90  
  7 purchase totals = $986.72       $1,091.2904   $1,091.29   $1,091.30   $1,091.40