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 $11.71   11.490%   $13.0555   $13.06   $13.05   $13.10  
  Purchase #2 $14.82   5.158%   $15.5844   $15.58   $15.60   $15.60  
  Purchase #3 $26.45   10.873%   $29.3259   $29.33   $29.35   $29.30  
  Purchase #4 $115.84   5.144%   $121.7988   $121.80   $121.80   $121.80  
  Purchase #5 $25.60   8.280%   $27.7197   $27.72   $27.70   $27.70  
  Purchase #6 $127.84   5.244%   $134.5439   $134.54   $134.55   $134.50  
  Purchase #7 $156.73   8.733%   $170.4172   $170.42   $170.40   $170.40  
  7 purchase totals = $478.99       $512.4454   $512.45   $512.45   $512.40