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 $19.99   9.671%   $21.9232   $21.92   $21.90   $21.90  
  Purchase #2 $28.77   6.391%   $30.6087   $30.61   $30.60   $30.60  
  Purchase #3 $181.29   8.703%   $197.0677   $197.07   $197.05   $197.10  
  Purchase #4 $111.40   13.279%   $126.1928   $126.19   $126.20   $126.20  
  Purchase #5 $22.39   11.578%   $24.9823   $24.98   $25.00   $25.00  
  Purchase #6 $107.57   12.314%   $120.8162   $120.82   $120.80   $120.80  
  Purchase #7 $81.70   11.190%   $90.8422   $90.84   $90.85   $90.80  
  7 purchase totals = $553.11       $612.4331   $612.43   $612.40   $612.40