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 $142.76   11.398%   $159.0318   $159.03   $159.05   $159.00  
  Purchase #2 $22.73   8.144%   $24.5811   $24.58   $24.60   $24.60  
  Purchase #3 $144.76   4.736%   $151.6158   $151.62   $151.60   $151.60  
  Purchase #4 $91.45   3.106%   $94.2904   $94.29   $94.30   $94.30  
  Purchase #5 $181.37   12.456%   $203.9614   $203.96   $203.95   $204.00  
  Purchase #6 $143.79   5.229%   $151.3088   $151.31   $151.30   $151.30  
  Purchase #7 $18.65   3.666%   $19.3337   $19.33   $19.35   $19.30  
  7 purchase totals = $745.51       $804.1230   $804.12   $804.15   $804.10