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 $17.35   8.399%   $18.8072   $18.81   $18.80   $18.80  
  Purchase #2 $12.27   10.310%   $13.535   $13.54   $13.55   $13.50  
  Purchase #3 $21.78   10.652%   $24.1   $24.10   $24.10   $24.10  
  Purchase #4 $25.36   9.107%   $27.6695   $27.67   $27.65   $27.70  
  Purchase #5 $152.98   11.158%   $170.0495   $170.05   $170.05   $170.00  
  Purchase #6 $134.53   13.730%   $153.001   $153.00   $153.00   $153.00  
  Purchase #7 $151.51   6.365%   $161.1536   $161.15   $161.15   $161.20  
  7 purchase totals = $515.78       $568.3158   $568.32   $568.30   $568.30