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 $29.55   4.568%   $30.8998   $30.90   $30.90   $30.90  
  Purchase #2 $47.28   5.576%   $49.9163   $49.92   $49.90   $49.90  
  Purchase #3 $213.72   9.375%   $233.7563   $233.76   $233.75   $233.80  
  Purchase #4 $212.56   4.481%   $222.0848   $222.08   $222.10   $222.10  
  Purchase #5 $57.45   4.645%   $60.1186   $60.12   $60.10   $60.10  
  Purchase #6 $12.44   3.430%   $12.8667   $12.87   $12.85   $12.90  
  Purchase #7 $29.48   5.542%   $31.1138   $31.11   $31.10   $31.10  
  7 purchase totals = $602.48       $640.7563   $640.76   $640.70   $640.80