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 $192.37   6.464%   $204.8048   $204.80   $204.80   $204.80  
  Purchase #2 $38.96   7.613%   $41.926   $41.93   $41.95   $41.90  
  Purchase #3 $78.63   3.314%   $81.2358   $81.24   $81.25   $81.20  
  Purchase #4 $155.57   6.392%   $165.514   $165.51   $165.50   $165.50  
  Purchase #5 $142.56   12.311%   $160.1106   $160.11   $160.10   $160.10  
  Purchase #6 $58.51   7.759%   $63.0498   $63.05   $63.05   $63.00  
  Purchase #7 $177.91   5.299%   $187.3375   $187.34   $187.35   $187.30  
  7 purchase totals = $844.51       $903.9785   $903.98   $904.00   $903.80