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 $12.40   3.758%   $12.866   $12.87   $12.85   $12.90  
  Purchase #2 $34.92   7.233%   $37.4458   $37.45   $37.45   $37.40  
  Purchase #3 $64.44   8.753%   $70.0804   $70.08   $70.10   $70.10  
  Purchase #4 $29.31   13.268%   $33.1989   $33.20   $33.20   $33.20  
  Purchase #5 $116.35   7.860%   $125.4951   $125.50   $125.50   $125.50  
  Purchase #6 $49.53   11.624%   $55.2874   $55.29   $55.30   $55.30  
  Purchase #7 $84.62   12.711%   $95.376   $95.38   $95.40   $95.40  
  7 purchase totals = $391.57       $429.7496   $429.77   $429.80   $429.80