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 $234.91   6.114%   $249.2724   $249.27   $249.25   $249.30  
  Purchase #2 $117.29   13.767%   $133.4373   $133.44   $133.45   $133.40  
  Purchase #3 $202.30   4.719%   $211.8465   $211.85   $211.85   $211.80  
  Purchase #4 $219.68   12.654%   $247.4783   $247.48   $247.50   $247.50  
  Purchase #5 $73.55   5.740%   $77.7718   $77.77   $77.75   $77.80  
  Purchase #6 $53.88   5.590%   $56.8919   $56.89   $56.90   $56.90  
  Purchase #7 $242.84   8.132%   $262.5877   $262.59   $262.60   $262.60  
  7 purchase totals = $1,144.45       $1,239.2859   $1,239.29   $1,239.30   $1,239.30