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 $156.87   8.840%   $170.7373   $170.74   $170.75   $170.70  
  Purchase #2 $111.37   13.801%   $126.7402   $126.74   $126.75   $126.70  
  Purchase #3 $120.53   8.656%   $130.9631   $130.96   $130.95   $131.00  
  Purchase #4 $148.66   12.507%   $167.2529   $167.25   $167.25   $167.30  
  Purchase #5 $127.29   11.796%   $142.3051   $142.31   $142.30   $142.30  
  Purchase #6 $31.88   13.466%   $36.173   $36.17   $36.15   $36.20  
  Purchase #7 $223.74   11.623%   $249.7453   $249.75   $249.75   $249.70  
  7 purchase totals = $920.34       $1,023.9169   $1,023.92   $1,023.90   $1,023.90