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 $30.37   4.647%   $31.7813   $31.78   $31.80   $31.80  
  Purchase #2 $235.59   3.645%   $244.1773   $244.18   $244.20   $244.20  
  Purchase #3 $9.97   4.474%   $10.4161   $10.42   $10.40   $10.40  
  Purchase #4 $46.39   7.239%   $49.7482   $49.75   $49.75   $49.70  
  Purchase #5 $183.65   7.747%   $197.8774   $197.88   $197.90   $197.90  
  Purchase #6 $9.58   3.400%   $9.9057   $9.91   $9.90   $9.90  
  Purchase #7 $9.84   9.774%   $10.8018   $10.80   $10.80   $10.80  
  7 purchase totals = $525.39       $554.7078   $554.72   $554.75   $554.70