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 $138.94   4.251%   $144.8463   $144.85   $144.85   $144.80  
  Purchase #2 $122.33   8.793%   $133.0865   $133.09   $133.10   $133.10  
  Purchase #3 $47.58   5.832%   $50.3549   $50.35   $50.35   $50.40  
  Purchase #4 $168.88   10.794%   $187.1089   $187.11   $187.10   $187.10  
  Purchase #5 $10.80   4.247%   $11.2587   $11.26   $11.25   $11.30  
  Purchase #6 $39.49   3.237%   $40.7683   $40.77   $40.75   $40.80  
  Purchase #7 $93.68   5.535%   $98.8652   $98.87   $98.85   $98.90  
  7 purchase totals = $621.70       $666.2888   $666.30   $666.25   $666.40