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 $164.83   12.242%   $185.0085   $185.01   $185.00   $185.00  
  Purchase #2 $149.92   6.523%   $159.6993   $159.70   $159.70   $159.70  
  Purchase #3 $117.75   9.406%   $128.8256   $128.83   $128.85   $128.80  
  Purchase #4 $28.45   4.194%   $29.6432   $29.64   $29.65   $29.60  
  Purchase #5 $231.82   11.656%   $258.8409   $258.84   $258.85   $258.80  
  Purchase #6 $238.77   11.334%   $265.8322   $265.83   $265.85   $265.80  
  Purchase #7 $49.94   12.177%   $56.0212   $56.02   $56.00   $56.00  
  7 purchase totals = $981.48       $1,083.8709   $1,083.87   $1,083.90   $1,083.70