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 $16.31   11.453%   $18.178   $18.18   $18.20   $18.20  
  Purchase #2 $23.47   6.820%   $25.0707   $25.07   $25.05   $25.10  
  Purchase #3 $184.75   8.307%   $200.0972   $200.10   $200.10   $200.10  
  Purchase #4 $35.65   7.548%   $38.3409   $38.34   $38.35   $38.30  
  Purchase #5 $177.70   8.165%   $192.2092   $192.21   $192.20   $192.20  
  Purchase #6 $158.55   4.323%   $165.4041   $165.40   $165.40   $165.40  
  Purchase #7 $24.30   10.510%   $26.8539   $26.85   $26.85   $26.90  
  7 purchase totals = $620.73       $666.1540   $666.15   $666.15   $666.20