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 $227.91   11.230%   $253.5043   $253.50   $253.50   $253.50  
  Purchase #2 $127.38   6.219%   $135.3018   $135.30   $135.30   $135.30  
  Purchase #3 $110.31   5.503%   $116.3804   $116.38   $116.40   $116.40  
  Purchase #4 $65.60   10.278%   $72.3424   $72.34   $72.35   $72.30  
  Purchase #5 $187.79   4.677%   $196.5729   $196.57   $196.55   $196.60  
  Purchase #6 $76.40   10.766%   $84.6252   $84.63   $84.65   $84.60  
  Purchase #7 $127.60   8.394%   $138.3107   $138.31   $138.30   $138.30  
  7 purchase totals = $922.99       $997.0377   $997.03   $997.05   $997.00