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 $249.34   13.564%   $283.1605   $283.16   $283.15   $283.20  
  Purchase #2 $180.60   11.642%   $201.6255   $201.63   $201.65   $201.60  
  Purchase #3 $158.26   12.654%   $178.2862   $178.29   $178.30   $178.30  
  Purchase #4 $235.68   5.311%   $248.197   $248.20   $248.20   $248.20  
  Purchase #5 $31.84   12.769%   $35.9056   $35.91   $35.90   $35.90  
  Purchase #6 $242.73   8.866%   $264.2504   $264.25   $264.25   $264.30  
  Purchase #7 $118.38   5.470%   $124.8554   $124.86   $124.85   $124.90  
  7 purchase totals = $1,216.83       $1,336.2806   $1,336.30   $1,336.30   $1,336.40