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 $12.33   6.320%   $13.1093   $13.11   $13.10   $13.10  
  Purchase #2 $217.45   8.304%   $235.507   $235.51   $235.50   $235.50  
  Purchase #3 $238.32   13.730%   $271.0413   $271.04   $271.05   $271.00  
  Purchase #4 $133.50   13.122%   $151.0179   $151.02   $151.00   $151.00  
  Purchase #5 $37.25   10.791%   $41.2696   $41.27   $41.25   $41.30  
  Purchase #6 $192.88   13.726%   $219.3547   $219.35   $219.35   $219.40  
  Purchase #7 $8.78   11.575%   $9.7963   $9.80   $9.80   $9.80  
  7 purchase totals = $840.51       $941.0961   $941.10   $941.05   $941.10