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 $75.48   7.822%   $81.384   $81.38   $81.40   $81.40  
  Purchase #2 $73.57   8.837%   $80.0714   $80.07   $80.05   $80.10  
  Purchase #3 $147.72   5.344%   $155.6142   $155.61   $155.60   $155.60  
  Purchase #4 $113.30   12.757%   $127.7537   $127.75   $127.75   $127.80  
  Purchase #5 $141.93   13.345%   $160.8706   $160.87   $160.85   $160.90  
  Purchase #6 $6.35   12.596%   $7.1498   $7.15   $7.15   $7.10  
  Purchase #7 $20.57   5.749%   $21.7526   $21.75   $21.75   $21.80  
  7 purchase totals = $578.92       $634.5963   $634.58   $634.55   $634.70