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 $5.79   10.519%   $6.3991   $6.40   $6.40   $6.40  
  Purchase #2 $166.40   5.525%   $175.5936   $175.59   $175.60   $175.60  
  Purchase #3 $19.45   9.446%   $21.2872   $21.29   $21.30   $21.30  
  Purchase #4 $100.54   12.248%   $112.8541   $112.85   $112.85   $112.90  
  Purchase #5 $74.64   7.657%   $80.3552   $80.36   $80.35   $80.40  
  Purchase #6 $124.98   8.289%   $135.3396   $135.34   $135.35   $135.30  
  Purchase #7 $15.49   8.593%   $16.8211   $16.82   $16.80   $16.80  
  7 purchase totals = $507.29       $548.6499   $548.65   $548.65   $548.70