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 $216.57   11.279%   $240.9969   $241.00   $241.00   $241.00  
  Purchase #2 $142.92   5.356%   $150.5748   $150.57   $150.55   $150.60  
  Purchase #3 $18.53   6.485%   $19.7317   $19.73   $19.75   $19.70  
  Purchase #4 $197.35   5.267%   $207.7444   $207.74   $207.75   $207.70  
  Purchase #5 $138.72   11.647%   $154.8767   $154.88   $154.90   $154.90  
  Purchase #6 $179.60   4.155%   $187.0624   $187.06   $187.05   $187.10  
  Purchase #7 $243.25   12.319%   $273.216   $273.22   $273.20   $273.20  
  7 purchase totals = $1,136.94       $1,234.2029   $1,234.20   $1,234.20   $1,234.20