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 $220.52   13.146%   $249.5096   $249.51   $249.50   $249.50  
  Purchase #2 $178.31   11.205%   $198.2896   $198.29   $198.30   $198.30  
  Purchase #3 $180.34   12.321%   $202.5597   $202.56   $202.55   $202.60  
  Purchase #4 $207.48   4.380%   $216.5676   $216.57   $216.55   $216.60  
  Purchase #5 $8.85   7.305%   $9.4965   $9.50   $9.50   $9.50  
  Purchase #6 $222.30   10.861%   $246.444   $246.44   $246.45   $246.40  
  Purchase #7 $140.34   4.259%   $146.3171   $146.32   $146.30   $146.30  
  7 purchase totals = $1,158.14       $1,269.1841   $1,269.19   $1,269.15   $1,269.20