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 $48.99   3.782%   $50.8428   $50.84   $50.85   $50.80  
  Purchase #2 $241.63   11.544%   $269.5238   $269.52   $269.50   $269.50  
  Purchase #3 $140.57   13.613%   $159.7058   $159.71   $159.70   $159.70  
  Purchase #4 $57.34   5.165%   $60.3016   $60.30   $60.30   $60.30  
  Purchase #5 $92.41   9.605%   $101.286   $101.29   $101.30   $101.30  
  Purchase #6 $241.84   12.211%   $271.3711   $271.37   $271.35   $271.40  
  Purchase #7 $224.71   10.183%   $247.5922   $247.59   $247.60   $247.60  
  7 purchase totals = $1,047.49       $1,160.6233   $1,160.62   $1,160.60   $1,160.60