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 $244.95   12.248%   $274.9515   $274.95   $274.95   $275.00  
  Purchase #2 $6.27   10.110%   $6.9039   $6.90   $6.90   $6.90  
  Purchase #3 $169.98   5.344%   $179.0637   $179.06   $179.05   $179.10  
  Purchase #4 $70.33   12.511%   $79.129   $79.13   $79.15   $79.10  
  Purchase #5 $95.56   10.454%   $105.5498   $105.55   $105.55   $105.50  
  Purchase #6 $29.89   4.282%   $31.1699   $31.17   $31.15   $31.20  
  Purchase #7 $82.26   10.699%   $91.061   $91.06   $91.05   $91.10  
  7 purchase totals = $699.24       $767.8288   $767.82   $767.80   $767.90