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 $34.93   13.556%   $39.6651   $39.67   $39.65   $39.70  
  Purchase #2 $121.87   13.160%   $137.9081   $137.91   $137.90   $137.90  
  Purchase #3 $61.96   9.804%   $68.0346   $68.03   $68.05   $68.00  
  Purchase #4 $167.42   8.119%   $181.0128   $181.01   $181.00   $181.00  
  Purchase #5 $211.53   12.717%   $238.4303   $238.43   $238.45   $238.40  
  Purchase #6 $118.73   4.654%   $124.2557   $124.26   $124.25   $124.30  
  Purchase #7 $248.27   5.342%   $261.5326   $261.53   $261.55   $261.50  
  7 purchase totals = $964.71       $1,050.8392   $1,050.84   $1,050.85   $1,050.80