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 $12.80   4.333%   $13.3546   $13.35   $13.35   $13.40  
  Purchase #2 $80.55   11.536%   $89.8422   $89.84   $89.85   $89.80  
  Purchase #3 $235.33   9.429%   $257.5193   $257.52   $257.50   $257.50  
  Purchase #4 $104.70   3.224%   $108.0755   $108.08   $108.10   $108.10  
  Purchase #5 $46.90   5.406%   $49.4354   $49.44   $49.45   $49.40  
  Purchase #6 $90.95   13.719%   $103.4274   $103.43   $103.45   $103.40  
  Purchase #7 $151.81   4.527%   $158.6824   $158.68   $158.70   $158.70  
  7 purchase totals = $723.04       $780.3368   $780.34   $780.40   $780.30