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 $45.45   12.356%   $51.0658   $51.07   $51.05   $51.10  
  Purchase #2 $12.64   3.580%   $13.0925   $13.09   $13.10   $13.10  
  Purchase #3 $144.29   9.339%   $157.7652   $157.77   $157.75   $157.80  
  Purchase #4 $140.38   13.510%   $159.3453   $159.35   $159.35   $159.30  
  Purchase #5 $9.72   5.852%   $10.2888   $10.29   $10.30   $10.30  
  Purchase #6 $147.90   4.632%   $154.7507   $154.75   $154.75   $154.80  
  Purchase #7 $25.67   5.631%   $27.1155   $27.12   $27.10   $27.10  
  7 purchase totals = $526.05       $573.4238   $573.44   $573.40   $573.50