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 $123.70   3.686%   $128.2596   $128.26   $128.25   $128.30  
  Purchase #2 $156.29   12.454%   $175.7544   $175.75   $175.75   $175.80  
  Purchase #3 $89.69   6.247%   $95.2929   $95.29   $95.30   $95.30  
  Purchase #4 $243.58   7.832%   $262.6572   $262.66   $262.65   $262.70  
  Purchase #5 $30.28   7.512%   $32.5546   $32.55   $32.55   $32.60  
  Purchase #6 $156.35   5.116%   $164.3489   $164.35   $164.35   $164.30  
  Purchase #7 $28.49   10.376%   $31.4461   $31.45   $31.45   $31.40  
  7 purchase totals = $828.38       $890.3137   $890.31   $890.30   $890.40