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 $17.31   9.235%   $18.9086   $18.91   $18.90   $18.90  
  Purchase #2 $109.26   5.261%   $115.0082   $115.01   $115.00   $115.00  
  Purchase #3 $171.70   5.424%   $181.013   $181.01   $181.00   $181.00  
  Purchase #4 $141.41   12.638%   $159.2814   $159.28   $159.30   $159.30  
  Purchase #5 $150.64   3.173%   $155.4198   $155.42   $155.40   $155.40  
  Purchase #6 $144.96   5.762%   $153.3126   $153.31   $153.30   $153.30  
  Purchase #7 $126.74   9.347%   $138.5864   $138.59   $138.60   $138.60  
  7 purchase totals = $862.02       $921.5300   $921.53   $921.50   $921.50