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 $45.85   12.672%   $51.6601   $51.66   $51.65   $51.70  
  Purchase #2 $232.66   10.151%   $256.2773   $256.28   $256.30   $256.30  
  Purchase #3 $185.34   7.811%   $199.8169   $199.82   $199.80   $199.80  
  Purchase #4 $249.52   11.515%   $278.2522   $278.25   $278.25   $278.30  
  Purchase #5 $178.56   8.745%   $194.1751   $194.18   $194.20   $194.20  
  Purchase #6 $62.35   4.327%   $65.0479   $65.05   $65.05   $65.00  
  Purchase #7 $103.45   10.222%   $114.0247   $114.02   $114.00   $114.00  
  7 purchase totals = $1,057.73       $1,159.2542   $1,159.26   $1,159.25   $1,159.30