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 $210.57   13.291%   $238.5569   $238.56   $238.55   $238.60  
  Purchase #2 $223.91   10.388%   $247.1698   $247.17   $247.15   $247.20  
  Purchase #3 $226.93   12.368%   $254.9967   $255.00   $255.00   $255.00  
  Purchase #4 $77.84   3.274%   $80.3885   $80.39   $80.40   $80.40  
  Purchase #5 $122.64   8.487%   $133.0485   $133.05   $133.05   $133.00  
  Purchase #6 $179.39   4.606%   $187.6527   $187.65   $187.65   $187.70  
  Purchase #7 $114.92   11.723%   $128.3921   $128.39   $128.40   $128.40  
  7 purchase totals = $1,156.20       $1,270.2052   $1,270.21   $1,270.20   $1,270.30