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 $250.33   11.617%   $279.4108   $279.41   $279.40   $279.40  
  Purchase #2 $217.27   10.424%   $239.9182   $239.92   $239.90   $239.90  
  Purchase #3 $153.27   12.777%   $172.8533   $172.85   $172.85   $172.90  
  Purchase #4 $103.64   12.644%   $116.7442   $116.74   $116.75   $116.70  
  Purchase #5 $98.61   11.207%   $109.6612   $109.66   $109.65   $109.70  
  Purchase #6 $212.80   9.766%   $233.582   $233.58   $233.60   $233.60  
  Purchase #7 $243.85   7.521%   $262.19   $262.19   $262.20   $262.20  
  7 purchase totals = $1,279.77       $1,414.3597   $1,414.35   $1,414.35   $1,414.40