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 $19.30   7.610%   $20.7687   $20.77   $20.75   $20.80  
  Purchase #2 $89.57   6.611%   $95.4915   $95.49   $95.50   $95.50  
  Purchase #3 $6.73   9.176%   $7.3475   $7.35   $7.35   $7.30  
  Purchase #4 $5.60   5.107%   $5.886   $5.89   $5.90   $5.90  
  Purchase #5 $39.60   4.428%   $41.3535   $41.35   $41.35   $41.40  
  Purchase #6 $128.25   13.231%   $145.2188   $145.22   $145.20   $145.20  
  Purchase #7 $124.87   6.219%   $132.6357   $132.64   $132.65   $132.60  
  7 purchase totals = $413.92       $448.7017   $448.71   $448.70   $448.70