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 $100.39   9.362%   $109.7885   $109.79   $109.80   $109.80  
  Purchase #2 $203.47   6.670%   $217.0414   $217.04   $217.05   $217.00  
  Purchase #3 $193.86   4.285%   $202.1669   $202.17   $202.15   $202.20  
  Purchase #4 $140.27   5.806%   $148.4141   $148.41   $148.40   $148.40  
  Purchase #5 $89.90   13.540%   $102.0725   $102.07   $102.05   $102.10  
  Purchase #6 $173.97   8.581%   $188.8984   $188.90   $188.90   $188.90  
  Purchase #7 $10.30   9.412%   $11.2694   $11.27   $11.25   $11.30  
  7 purchase totals = $912.16       $979.6512   $979.65   $979.60   $979.70