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 $70.53   13.577%   $80.1059   $80.11   $80.10   $80.10  
  Purchase #2 $127.29   6.807%   $135.9546   $135.95   $135.95   $136.00  
  Purchase #3 $145.96   11.600%   $162.8914   $162.89   $162.90   $162.90  
  Purchase #4 $167.47   3.495%   $173.3231   $173.32   $173.30   $173.30  
  Purchase #5 $192.96   7.105%   $206.6698   $206.67   $206.65   $206.70  
  Purchase #6 $160.76   5.465%   $169.5455   $169.55   $169.55   $169.50  
  Purchase #7 $198.98   12.247%   $223.3491   $223.35   $223.35   $223.30  
  7 purchase totals = $1,063.95       $1,151.8394   $1,151.84   $1,151.80   $1,151.80