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 $222.75   12.208%   $249.9433   $249.94   $249.95   $249.90  
  Purchase #2 $177.34   13.776%   $201.7704   $201.77   $201.75   $201.80  
  Purchase #3 $118.88   4.454%   $124.1749   $124.17   $124.15   $124.20  
  Purchase #4 $86.51   3.627%   $89.6477   $89.65   $89.65   $89.60  
  Purchase #5 $24.85   7.243%   $26.6499   $26.65   $26.65   $26.60  
  Purchase #6 $19.27   11.430%   $21.4726   $21.47   $21.45   $21.50  
  Purchase #7 $86.39   7.199%   $92.6092   $92.61   $92.60   $92.60  
  7 purchase totals = $735.99       $806.2680   $806.26   $806.20   $806.20