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 $240.97   12.753%   $271.7009   $271.70   $271.70   $271.70  
  Purchase #2 $25.89   3.348%   $26.7568   $26.76   $26.75   $26.80  
  Purchase #3 $211.58   4.555%   $221.2175   $221.22   $221.20   $221.20  
  Purchase #4 $176.65   10.594%   $195.3643   $195.36   $195.35   $195.40  
  Purchase #5 $143.74   9.481%   $157.368   $157.37   $157.35   $157.40  
  Purchase #6 $45.70   12.724%   $51.5149   $51.51   $51.50   $51.50  
  Purchase #7 $86.38   8.428%   $93.6601   $93.66   $93.65   $93.70  
  7 purchase totals = $930.91       $1,017.5825   $1,017.58   $1,017.50   $1,017.70