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 $7.80   13.312%   $8.8383   $8.84   $8.85   $8.80  
  Purchase #2 $51.61   8.761%   $56.1316   $56.13   $56.15   $56.10  
  Purchase #3 $18.52   7.275%   $19.8673   $19.87   $19.85   $19.90  
  Purchase #4 $190.58   8.191%   $206.1904   $206.19   $206.20   $206.20  
  Purchase #5 $38.88   4.579%   $40.6603   $40.66   $40.65   $40.70  
  Purchase #6 $56.93   10.341%   $62.8171   $62.82   $62.80   $62.80  
  Purchase #7 $133.51   11.537%   $148.913   $148.91   $148.90   $148.90  
  7 purchase totals = $497.83       $543.4180   $543.42   $543.40   $543.40