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 $19.29   8.787%   $20.985   $20.99   $21.00   $21.00  
  Purchase #2 $202.51   10.826%   $224.4337   $224.43   $224.45   $224.40  
  Purchase #3 $197.26   12.136%   $221.1995   $221.20   $221.20   $221.20  
  Purchase #4 $112.35   8.560%   $121.9672   $121.97   $121.95   $122.00  
  Purchase #5 $228.38   9.133%   $249.2379   $249.24   $249.25   $249.20  
  Purchase #6 $223.75   13.578%   $254.1308   $254.13   $254.15   $254.10  
  Purchase #7 $16.55   9.558%   $18.1318   $18.13   $18.15   $18.10  
  7 purchase totals = $1,000.09       $1,110.0859   $1,110.09   $1,110.15   $1,110.00