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 $248.87   12.406%   $279.7448   $279.74   $279.75   $279.70  
  Purchase #2 $247.80   8.473%   $268.7961   $268.80   $268.80   $268.80  
  Purchase #3 $65.51   11.760%   $73.214   $73.21   $73.20   $73.20  
  Purchase #4 $81.62   3.617%   $84.5722   $84.57   $84.55   $84.60  
  Purchase #5 $149.83   3.682%   $155.3467   $155.35   $155.35   $155.30  
  Purchase #6 $157.47   10.173%   $173.4894   $173.49   $173.50   $173.50  
  Purchase #7 $161.52   3.160%   $166.624   $166.62   $166.60   $166.60  
  7 purchase totals = $1,112.62       $1,201.7872   $1,201.78   $1,201.75   $1,201.70