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 $240.49   13.726%   $273.4997   $273.50   $273.50   $273.50  
  Purchase #2 $206.33   6.413%   $219.5619   $219.56   $219.55   $219.60  
  Purchase #3 $136.98   7.309%   $146.9919   $146.99   $147.00   $147.00  
  Purchase #4 $68.27   12.616%   $76.8829   $76.88   $76.90   $76.90  
  Purchase #5 $64.31   8.463%   $69.7526   $69.75   $69.75   $69.80  
  Purchase #6 $190.97   3.723%   $198.0798   $198.08   $198.10   $198.10  
  Purchase #7 $58.49   3.293%   $60.4161   $60.42   $60.40   $60.40  
  7 purchase totals = $965.84       $1,045.1849   $1,045.18   $1,045.20   $1,045.30