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 $137.90   13.514%   $156.5358   $156.54   $156.55   $156.50  
  Purchase #2 $43.37   12.847%   $48.9417   $48.94   $48.95   $48.90  
  Purchase #3 $116.68   7.439%   $125.3598   $125.36   $125.35   $125.40  
  Purchase #4 $90.27   10.287%   $99.5561   $99.56   $99.55   $99.60  
  Purchase #5 $20.59   11.301%   $22.9169   $22.92   $22.90   $22.90  
  Purchase #6 $83.84   5.192%   $88.193   $88.19   $88.20   $88.20  
  Purchase #7 $192.65   6.545%   $205.2589   $205.26   $205.25   $205.30  
  7 purchase totals = $685.30       $746.7622   $746.77   $746.75   $746.80