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 $248.64   11.290%   $276.7115   $276.71   $276.70   $276.70  
  Purchase #2 $116.78   12.569%   $131.4581   $131.46   $131.45   $131.50  
  Purchase #3 $115.91   4.191%   $120.7678   $120.77   $120.75   $120.80  
  Purchase #4 $68.71   10.529%   $75.9445   $75.94   $75.95   $75.90  
  Purchase #5 $46.31   7.150%   $49.6212   $49.62   $49.60   $49.60  
  Purchase #6 $11.61   10.321%   $12.8083   $12.81   $12.80   $12.80  
  Purchase #7 $125.94   6.507%   $134.1349   $134.13   $134.15   $134.10  
  7 purchase totals = $733.90       $801.4463   $801.44   $801.40   $801.40