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 $241.66   12.406%   $271.6403   $271.64   $271.65   $271.60  
  Purchase #2 $173.38   6.392%   $184.4624   $184.46   $184.45   $184.50  
  Purchase #3 $65.97   3.606%   $68.3489   $68.35   $68.35   $68.30  
  Purchase #4 $39.56   5.273%   $41.646   $41.65   $41.65   $41.60  
  Purchase #5 $195.38   7.274%   $209.5919   $209.59   $209.60   $209.60  
  Purchase #6 $177.32   8.624%   $192.6121   $192.61   $192.60   $192.60  
  Purchase #7 $91.99   7.595%   $98.9766   $98.98   $99.00   $99.00  
  7 purchase totals = $985.26       $1,067.2782   $1,067.28   $1,067.30   $1,067.20