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 $19.43   6.434%   $20.6801   $20.68   $20.70   $20.70  
  Purchase #2 $12.53   6.334%   $13.3237   $13.32   $13.30   $13.30  
  Purchase #3 $40.77   9.363%   $44.5873   $44.59   $44.60   $44.60  
  Purchase #4 $39.91   12.745%   $44.9965   $45.00   $45.00   $45.00  
  Purchase #5 $30.96   11.125%   $34.4043   $34.40   $34.40   $34.40  
  Purchase #6 $39.32   4.162%   $40.9565   $40.96   $40.95   $41.00  
  Purchase #7 $130.34   13.808%   $148.3373   $148.34   $148.35   $148.30  
  7 purchase totals = $313.26       $347.2857   $347.29   $347.30   $347.30