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 $56.80   12.785%   $64.0619   $64.06   $64.05   $64.10  
  Purchase #2 $194.79   8.647%   $211.6335   $211.63   $211.65   $211.60  
  Purchase #3 $29.39   5.693%   $31.0632   $31.06   $31.05   $31.10  
  Purchase #4 $217.41   12.483%   $244.5493   $244.55   $244.55   $244.50  
  Purchase #5 $185.57   10.323%   $204.7264   $204.73   $204.75   $204.70  
  Purchase #6 $126.30   7.138%   $135.3153   $135.32   $135.30   $135.30  
  Purchase #7 $176.91   8.494%   $191.9367   $191.94   $191.95   $191.90  
  7 purchase totals = $987.17       $1,083.2863   $1,083.29   $1,083.30   $1,083.20