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 $121.63   11.794%   $135.975   $135.98   $136.00   $136.00  
  Purchase #2 $5.47   13.615%   $6.2147   $6.21   $6.20   $6.20  
  Purchase #3 $224.88   12.414%   $252.7966   $252.80   $252.80   $252.80  
  Purchase #4 $59.37   7.591%   $63.8768   $63.88   $63.90   $63.90  
  Purchase #5 $241.68   7.440%   $259.661   $259.66   $259.65   $259.70  
  Purchase #6 $86.90   11.823%   $97.1742   $97.17   $97.15   $97.20  
  Purchase #7 $183.72   11.107%   $204.1258   $204.13   $204.15   $204.10  
  7 purchase totals = $923.65       $1,019.8241   $1,019.83   $1,019.85   $1,019.90