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 $19.56   4.431%   $20.4267   $20.43   $20.45   $20.40  
  Purchase #2 $7.64   11.630%   $8.5285   $8.53   $8.55   $8.50  
  Purchase #3 $188.45   12.258%   $211.5502   $211.55   $211.55   $211.60  
  Purchase #4 $135.51   3.784%   $140.6377   $140.64   $140.65   $140.60  
  Purchase #5 $11.43   3.702%   $11.8531   $11.85   $11.85   $11.90  
  Purchase #6 $153.48   9.315%   $167.7767   $167.78   $167.80   $167.80  
  Purchase #7 $37.29   7.441%   $40.0647   $40.06   $40.05   $40.10  
  7 purchase totals = $553.36       $600.8376   $600.84   $600.90   $600.90