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 $227.88   9.831%   $250.2829   $250.28   $250.30   $250.30  
  Purchase #2 $112.91   3.487%   $116.8472   $116.85   $116.85   $116.80  
  Purchase #3 $67.53   11.306%   $75.1649   $75.16   $75.15   $75.20  
  Purchase #4 $173.55   11.549%   $193.5933   $193.59   $193.60   $193.60  
  Purchase #5 $27.41   11.466%   $30.5528   $30.55   $30.55   $30.60  
  Purchase #6 $203.50   8.852%   $221.5138   $221.51   $221.50   $221.50  
  Purchase #7 $32.94   9.293%   $36.0011   $36.00   $36.00   $36.00  
  7 purchase totals = $845.72       $923.9560   $923.94   $923.95   $924.00