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 $93.87   11.131%   $104.3187   $104.32   $104.30   $104.30  
  Purchase #2 $25.54   8.478%   $27.7053   $27.71   $27.70   $27.70  
  Purchase #3 $117.97   3.230%   $121.7804   $121.78   $121.80   $121.80  
  Purchase #4 $75.90   6.672%   $80.964   $80.96   $80.95   $81.00  
  Purchase #5 $68.32   8.537%   $74.1525   $74.15   $74.15   $74.20  
  Purchase #6 $60.93   5.844%   $64.4907   $64.49   $64.50   $64.50  
  Purchase #7 $42.98   9.257%   $46.9587   $46.96   $46.95   $47.00  
  7 purchase totals = $485.51       $520.3703   $520.37   $520.35   $520.50