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 $165.95   11.746%   $185.4425   $185.44   $185.45   $185.40  
  Purchase #2 $189.33   9.301%   $206.9396   $206.94   $206.95   $206.90  
  Purchase #3 $113.30   11.521%   $126.3533   $126.35   $126.35   $126.40  
  Purchase #4 $165.80   4.202%   $172.7669   $172.77   $172.75   $172.80  
  Purchase #5 $184.45   5.834%   $195.2108   $195.21   $195.20   $195.20  
  Purchase #6 $249.49   9.373%   $272.8747   $272.87   $272.85   $272.90  
  Purchase #7 $79.42   11.471%   $88.5303   $88.53   $88.55   $88.50  
  7 purchase totals = $1,147.74       $1,248.1181   $1,248.11   $1,248.10   $1,248.10