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 $18.26   3.391%   $18.8792   $18.88   $18.90   $18.90  
  Purchase #2 $66.59   3.658%   $69.0259   $69.03   $69.05   $69.00  
  Purchase #3 $128.87   5.728%   $136.2517   $136.25   $136.25   $136.30  
  Purchase #4 $18.37   10.447%   $20.2891   $20.29   $20.30   $20.30  
  Purchase #5 $93.96   4.352%   $98.0491   $98.05   $98.05   $98.00  
  Purchase #6 $123.72   11.751%   $138.2583   $138.26   $138.25   $138.30  
  Purchase #7 $140.97   12.763%   $158.962   $158.96   $158.95   $159.00  
  7 purchase totals = $590.74       $639.7153   $639.72   $639.75   $639.80