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 $227.47   6.178%   $241.5231   $241.52   $241.50   $241.50  
  Purchase #2 $106.81   13.475%   $121.2026   $121.20   $121.20   $121.20  
  Purchase #3 $138.37   3.585%   $143.3306   $143.33   $143.35   $143.30  
  Purchase #4 $18.79   9.445%   $20.5647   $20.56   $20.55   $20.60  
  Purchase #5 $190.60   11.189%   $211.9262   $211.93   $211.95   $211.90  
  Purchase #6 $249.49   9.630%   $273.5159   $273.52   $273.50   $273.50  
  Purchase #7 $93.25   10.614%   $103.1476   $103.15   $103.15   $103.10  
  7 purchase totals = $1,024.78       $1,115.2107   $1,115.21   $1,115.20   $1,115.10