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 $108.47   11.395%   $120.8302   $120.83   $120.85   $120.80  
  Purchase #2 $96.40   12.871%   $108.8076   $108.81   $108.80   $108.80  
  Purchase #3 $212.71   10.179%   $234.3618   $234.36   $234.35   $234.40  
  Purchase #4 $128.26   7.687%   $138.1193   $138.12   $138.10   $138.10  
  Purchase #5 $20.60   8.331%   $22.3162   $22.32   $22.30   $22.30  
  Purchase #6 $183.28   4.249%   $191.0676   $191.07   $191.05   $191.10  
  Purchase #7 $94.72   5.481%   $99.9116   $99.91   $99.90   $99.90  
  7 purchase totals = $844.44       $915.4143   $915.42   $915.35   $915.40