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 $231.50   11.567%   $258.2776   $258.28   $258.30   $258.30  
  Purchase #2 $215.93   6.457%   $229.8726   $229.87   $229.85   $229.90  
  Purchase #3 $72.93   8.653%   $79.2406   $79.24   $79.25   $79.20  
  Purchase #4 $29.74   13.726%   $33.8221   $33.82   $33.80   $33.80  
  Purchase #5 $24.54   3.815%   $25.4762   $25.48   $25.50   $25.50  
  Purchase #6 $137.69   3.136%   $142.008   $142.01   $142.00   $142.00  
  Purchase #7 $202.71   11.673%   $226.3723   $226.37   $226.35   $226.40  
  7 purchase totals = $915.04       $995.0694   $995.07   $995.05   $995.10