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 $234.53   6.243%   $249.1717   $249.17   $249.15   $249.20  
  Purchase #2 $190.79   5.424%   $201.1384   $201.14   $201.15   $201.10  
  Purchase #3 $111.50   8.833%   $121.3488   $121.35   $121.35   $121.30  
  Purchase #4 $167.26   9.332%   $182.8687   $182.87   $182.85   $182.90  
  Purchase #5 $34.74   13.181%   $39.3191   $39.32   $39.30   $39.30  
  Purchase #6 $62.81   6.302%   $66.7683   $66.77   $66.75   $66.80  
  Purchase #7 $142.41   7.439%   $153.0039   $153.00   $153.00   $153.00  
  7 purchase totals = $944.04       $1,013.6189   $1,013.62   $1,013.55   $1,013.60