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 $58.45   13.103%   $66.1087   $66.11   $66.10   $66.10  
  Purchase #2 $132.87   10.366%   $146.6433   $146.64   $146.65   $146.60  
  Purchase #3 $222.26   11.562%   $247.9577   $247.96   $247.95   $248.00  
  Purchase #4 $13.84   12.811%   $15.613   $15.61   $15.60   $15.60  
  Purchase #5 $232.35   10.234%   $256.1287   $256.13   $256.15   $256.10  
  Purchase #6 $179.28   11.308%   $199.553   $199.55   $199.55   $199.60  
  Purchase #7 $134.87   8.674%   $146.5686   $146.57   $146.55   $146.60  
  7 purchase totals = $973.92       $1,078.5730   $1,078.57   $1,078.55   $1,078.60