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 $106.53   13.712%   $121.1374   $121.14   $121.15   $121.10  
  Purchase #2 $97.39   6.479%   $103.6999   $103.70   $103.70   $103.70  
  Purchase #3 $212.49   6.821%   $226.9839   $226.98   $227.00   $227.00  
  Purchase #4 $96.51   6.186%   $102.4801   $102.48   $102.50   $102.50  
  Purchase #5 $166.49   3.178%   $171.7811   $171.78   $171.80   $171.80  
  Purchase #6 $46.89   6.707%   $50.0349   $50.03   $50.05   $50.00  
  Purchase #7 $199.76   10.221%   $220.1775   $220.18   $220.20   $220.20  
  7 purchase totals = $926.06       $996.2948   $996.29   $996.40   $996.30