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 $37.77   11.342%   $42.0539   $42.05   $42.05   $42.10  
  Purchase #2 $6.92   5.211%   $7.2806   $7.28   $7.30   $7.30  
  Purchase #3 $8.85   6.491%   $9.4245   $9.42   $9.40   $9.40  
  Purchase #4 $224.30   10.154%   $247.0754   $247.08   $247.10   $247.10  
  Purchase #5 $110.54   6.392%   $117.6057   $117.61   $117.60   $117.60  
  Purchase #6 $105.33   9.413%   $115.2447   $115.24   $115.25   $115.20  
  Purchase #7 $124.94   8.605%   $135.6911   $135.69   $135.70   $135.70  
  7 purchase totals = $618.65       $674.3759   $674.37   $674.40   $674.40