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 $249.74   13.851%   $284.3315   $284.33   $284.35   $284.30  
  Purchase #2 $158.94   10.659%   $175.8814   $175.88   $175.90   $175.90  
  Purchase #3 $170.35   13.858%   $193.9571   $193.96   $193.95   $194.00  
  Purchase #4 $79.28   12.755%   $89.3922   $89.39   $89.40   $89.40  
  Purchase #5 $171.61   13.437%   $194.6692   $194.67   $194.65   $194.70  
  Purchase #6 $232.86   3.784%   $241.6714   $241.67   $241.65   $241.70  
  Purchase #7 $168.77   11.534%   $188.2359   $188.24   $188.25   $188.20  
  7 purchase totals = $1,231.55       $1,368.1387   $1,368.14   $1,368.15   $1,368.20