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 $133.82   3.488%   $138.4876   $138.49   $138.50   $138.50  
  Purchase #2 $167.25   9.410%   $182.9882   $182.99   $183.00   $183.00  
  Purchase #3 $179.58   3.470%   $185.8114   $185.81   $185.80   $185.80  
  Purchase #4 $249.26   7.827%   $268.7696   $268.77   $268.75   $268.80  
  Purchase #5 $228.43   7.260%   $245.014   $245.01   $245.00   $245.00  
  Purchase #6 $224.29   6.458%   $238.7746   $238.77   $238.75   $238.80  
  Purchase #7 $246.33   12.169%   $276.3059   $276.31   $276.30   $276.30  
  7 purchase totals = $1,428.96       $1,536.1513   $1,536.15   $1,536.10   $1,536.20