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 $49.41   13.824%   $56.2404   $56.24   $56.25   $56.20  
  Purchase #2 $177.91   11.823%   $198.9443   $198.94   $198.95   $198.90  
  Purchase #3 $68.39   9.700%   $75.0238   $75.02   $75.00   $75.00  
  Purchase #4 $107.96   10.415%   $119.204   $119.20   $119.20   $119.20  
  Purchase #5 $247.77   5.488%   $261.3676   $261.37   $261.35   $261.40  
  Purchase #6 $60.69   12.175%   $68.079   $68.08   $68.10   $68.10  
  Purchase #7 $136.68   3.540%   $141.5185   $141.52   $141.50   $141.50  
  7 purchase totals = $848.81       $920.3776   $920.37   $920.35   $920.30