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 $135.97   13.699%   $154.5965   $154.60   $154.60   $154.60  
  Purchase #2 $211.86   5.423%   $223.3492   $223.35   $223.35   $223.30  
  Purchase #3 $185.65   12.226%   $208.3476   $208.35   $208.35   $208.30  
  Purchase #4 $198.36   4.313%   $206.9153   $206.92   $206.90   $206.90  
  Purchase #5 $165.87   10.470%   $183.2366   $183.24   $183.25   $183.20  
  Purchase #6 $244.54   12.717%   $275.6382   $275.64   $275.65   $275.60  
  Purchase #7 $91.38   7.150%   $97.9137   $97.91   $97.90   $97.90  
  7 purchase totals = $1,233.63       $1,349.9971   $1,350.01   $1,350.00   $1,349.80