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 $230.46   12.367%   $258.961   $258.96   $258.95   $259.00  
  Purchase #2 $32.80   6.608%   $34.9674   $34.97   $34.95   $35.00  
  Purchase #3 $148.90   8.278%   $161.2259   $161.23   $161.25   $161.20  
  Purchase #4 $216.92   5.360%   $228.5469   $228.55   $228.55   $228.50  
  Purchase #5 $97.42   7.772%   $104.9915   $104.99   $105.00   $105.00  
  Purchase #6 $48.39   11.675%   $54.0395   $54.04   $54.05   $54.00  
  Purchase #7 $209.59   11.717%   $234.1477   $234.15   $234.15   $234.10  
  7 purchase totals = $984.48       $1,076.8799   $1,076.89   $1,076.90   $1,076.80