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 $20.74   10.638%   $22.9463   $22.95   $22.95   $22.90  
  Purchase #2 $6.49   9.491%   $7.106   $7.11   $7.10   $7.10  
  Purchase #3 $178.49   9.160%   $194.8397   $194.84   $194.85   $194.80  
  Purchase #4 $156.34   3.198%   $161.3398   $161.34   $161.35   $161.30  
  Purchase #5 $102.90   12.176%   $115.4291   $115.43   $115.45   $115.40  
  Purchase #6 $79.51   13.697%   $90.4005   $90.40   $90.40   $90.40  
  Purchase #7 $115.93   10.778%   $128.4249   $128.42   $128.40   $128.40  
  7 purchase totals = $660.40       $720.4863   $720.49   $720.50   $720.30